2002 Prius High-Pitched Howl Noise at speed

kocho · Oct 18, 2007

Between 28 and 38 miles per hour I hear a faint high-pitched noise that changes the frequency as I accelerate or slow-down. Sounds like electric motor winding up or down but I am not sure. It is different from the noise the generator does when regenerating. Audible when in electric only or in combined mode. Not audible under even mmoderate acceleration as the engine noise then overshadows it.

Any thoughts? 118K on the Prius, just bought it used and I did not seem to notice this noise before (drove it for about 400 miles so far. Perhaps I'm getting more sensitive to the noises it makes, but want to make sure.

The gage shows me average of 54 to 55 mpg over the last 400 miles Washington Metro DC traffic so no complaints about the fuel economy...

drifty1955 · Oct 18, 2007

<div class='quotetop'>QUOTE(Kocho @ Oct 18 2007, 09:49 AM) [snapback]527278[/snapback]</div>

Between 28 and 38 miles per hour I hear a faint high-pitched noise that changes the frequency as I accelerate or slow-down. Sounds like electric motor winding up or down but I am not sure. It is different from the noise the generator does when regenerating. Audible when in electric only or in combined mode. Not audible under even mmoderate acceleration as the engine noise then overshadows it.

Any thoughts? 118K on the Prius, just bought it used and I did not seem to notice this noise before (drove it for about 400 miles so far. Perhaps I'm getting more sensitive to the noises it makes, but want to make sure.

The gage shows me average of 54 to 55 mpg over the last 400 miles Washington Metro DC traffic so no complaints about the fuel economy...
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What noise does the generator make when its regenerating? Never heard that noise before. I'm confused...your header states its a 2008 and your saying it has 118K on it? WOW!! Anyway I hope its not the dreaded humming of a failed M/G stator. Please search this. Its not good. Or maybe its just worn out brake pads up front and your hearing the scraper.You know the scraper that's there to alert clueless people that there pads are worn out and screeches when people go 10K more than they should on worn out pads and the pads/rivets are about to eat the rotors up. I'm sure our car has them. Never seen a car without them. Look at the rotors. There easy to see. Are they super deep worn out? Does the sound change when your coasting and then with the windows open listening apply the brakes. If the squeal goes away when you apply brake its brake time!!! Good luck!

donee · Oct 18, 2007

Hi Kocho,

Have you ever had the transmission fluid changed ? That could be the bearings in the power sharing device in the transmission. The first generation cars used sleeve bearings, and the newer generation use needle bearings. Look around for Robert J Wilsons posts regarding the first generation transmission.

Hi Again Kocho,

You might also check the water pump, idler bearing (if the gen one has one) or A/C compressor bearing. Would be somewhat cheaper than the transmission work. Good Luck!

kocho · Oct 19, 2007

The more I listen to it, the more it seems to be electrical related. The pitch changes somewhat independently of the RPM, contrary to what I first wrote. The noise resembles the sound an electrical vehicle like a subway or airport train make when accelerating and decellerating. Tt is definitely linked to the load on the electric motor but I am not sure if it is the motor doing it or the transmission or something else. Again, not a loud noise at all and is most noticeable around 35-38 miles per hour with the gas engine off and trying to accelerate on electric power alone (try that downhill so that the gas engine does not kick-in too easily).

To answer some of the questions in the posts here:
- This is a 2002 Prius. I need to update my profile - I think I goofed when my first submission failed and forgot to change the vehicle back to '02.

- The tranny oil I just changed last week, including dropping the pan and a new pan gasket. Some thickening on the pan bottom and magnet material but the oil had been changed some time before I bought the car and was in very good condition. Not sure what it was before that - probably very bad is my suspicion.

- The generator noise that someone said they've never heard is in my opinion pretty quiet but clearly audible in my case. It is best heard when I coast down and apply the brakes lightly and brake thru regeneration. It is the noise a dynamo makes when rotating and generating current. This generator noise is a little lower pitched than the noise I am talking about but both seem to come from the sam area physically - in the cabin I hear it coming from the front right (note that the transmission is on the front left) but that may be misleading, given all the ways sound can travel...

VTMRGRTVL · Oct 19, 2007

One of the front wheel bearings?

My suggestion would be to get a mechanic to go for a ride with you - Most will do that for free...

bwilson4web · Oct 19, 2007

<div class='quotetop'>QUOTE(Kocho @ Oct 19 2007, 10:04 AM) [snapback]527740[/snapback]</div>

The more I listen to it, the more it seems to be electrical related. The pitch changes somewhat independently of the RPM, contrary to what I first wrote. The noise resembles the sound an electrical vehicle like a subway or airport train make when accelerating and decellerating. Tt is definitely linked to the load on the electric motor but I am not sure if it is the motor doing it or the transmission or something else. Again, not a loud noise at all and is most noticeable around 35-38 miles per hour with the gas engine off and trying to accelerate on electric power alone (try that downhill so that the gas engine does not kick-in too easily).
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MG1 has a much higher rpm than the ICE and MG2, the main traction motor. As you approach 35-38 mph, you are coming into the range of maximum MG1 rpm.

Can you find a fairly steep hill to descend at speed (aka., I-66 into Arlington or from MD-to-VA over the north side of the Potomac?

Two tests I would be interested in:

1) Descend the hill starting at 38 mph in "D" and hopefully let gravity accelerate the car to +42 mph. We need to know if the sound goes away when the ICE begins spinning. MG1 rotation will approach maximum rpm as the ICE is off and the speed approaches 42 mph. This can help identify it as the source.

2) Descend the hill starting at 38 mph in "N" and hopefully let gravity accelerate the car to ~45 mph. We need to know if the sound goes away when the inverter is "out of the circuit." Try not to exceed 45 mph by using your brakes and slip back into "D" to see what happens.

The MG1 max rpm is 6,500 and it would be interesting if the noise is in this speed range.

There is a failure mode we are looking at that involves a coil on the MG overheating and becoming a 'disconnected' coil. This is bad because nothing the operator can do will have any effect on this progressive failure mode. We've seen it with MG2 but there is nothing that says MG1 couldn't have a problem.

<div class='quotetop'>QUOTE(Kocho @ Oct 19 2007, 10:04 AM) [snapback]527740[/snapback]</div>

To answer some of the questions in the posts here:
- This is a 2002 Prius. I need to update my profile - I think I goofed when my first submission failed and forgot to change the vehicle back to '02.

- The tranny oil I just changed last week, including dropping the pan and a new pan gasket. Some thickening on the pan bottom and magnet material but the oil had been changed some time before I bought the car and was in very good condition. Not sure what it was before that - probably very bad is my suspicion.

- The generator noise that someone said they've never heard is in my opinion pretty quiet but clearly audible in my case. It is best heard when I coast down and apply the brakes lightly and brake thru regeneration. It is the noise a dynamo makes when rotating and generating current. This generator noise is a little lower pitched than the noise I am talking about but both seem to come from the sam area physically - in the cabin I hear it coming from the front right (note that the transmission is on the front left) but that may be misleading, given all the ways sound can travel...
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Good news on the transaxle oil but I'm wondering if you managed to keep a sample? Samples don't have to be tested but having one makes that an option.

Wheel bearing go out but often they reveal themselves in turns. The sound will change if you turn left or right.

Bob Wilson

kocho · Oct 20, 2007

<div class='quotetop'>QUOTE(bwilson4web @ Oct 19 2007, 01:49 PM) [snapback]527816[/snapback]</div>

MG1 has a much higher rpm than the ICE and MG2, the main traction motor. As you approach 35-38 mph, you are coming into the range of maximum MG1 rpm.

Can you find a fairly steep hill to descend at speed (aka., I-66 into Arlington or from MD-to-VA over the north side of the Potomac?

Two tests I would be interested in:

1) Descend the hill starting at 38 mph in "D" and hopefully let gravity accelerate the car to +42 mph. We need to know if the sound goes away when the ICE begins spinning. MG1 rotation will approach maximum rpm as the ICE is off and the speed approaches 42 mph. This can help identify it as the source.

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I tried that in less than ideal circumstances yesterday and I am not sure I can make a conclusion. It seems that the noise remains even above 41 mph but then other noises (road noise, wind noise, other traffic) become stronger than it so I am not sure if it disappears or is just overcome by the other noises. As I said, it is not that loud.

2) Descend the hill starting at 38 mph in "N" and hopefully let gravity accelerate the car to ~45 mph. We need to know if the sound goes away when the inverter is "out of the circuit." Try not to exceed 45 mph by using your brakes and slip back into "D" to see what happens.

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I did not get a chance to to that as I did not come across a suitable hill. But when I just shift into N while I'm driving on electric only or on combined gas+ electric at around 38 miles (or any other speed), the noise disappears. But the problem again is that I only hear the noise in the first place if I put a little acceleration thru the electric motor. If I just coast, then I do not hear it...

The MG1 max rpm is 6,500 and it would be interesting if the noise is in this speed range.
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It very well might be that at that range it becomes more audible, but it actually starts when I drive slow, say 8 mph and sounds like whee-uuu and then becomes more audible and the loudest at about 38 mph or so (may be above that, but as I said other noises drown it thereafter so it is hard to tell). It is there regardless of whether the gas is on or off - only depends on if I press the accelerator to speed-up with the help of the electric motor...

There is a failure mode we are looking at that involves a coil on the MG overheating and becoming a 'disconnected' coil. This is bad because nothing the operator can do will have any effect on this progressive failure mode. We've seen it with MG2 but there is nothing that says MG1 couldn't have a problem.

Good news on the transaxle oil but I'm wondering if you managed to keep a sample? Samples don't have to be tested but having one makes that an option.
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The oil had apparently been chaged at some point, and it was in good condition - pretty good red color and no burnt smell. It would not have been of any use IMO as it was fres enough. Not sure if the few spoonfulls of gunk around the magnet and bottom of the tranny pan would have been of any use, but I did not keep these either.

Wheel bearing go out but often they reveal themselves in turns. The sound will change if you turn left or right.

Bob Wilson
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I think it is not a wheel bearing almost certainly as the noise is clearly correlated to the operation of the car with the help of the electric motor.

Thanks for the suggestions and I will try them again when I drive down hill next time.

bwilson4web · Oct 20, 2007

<div class='quotetop'>QUOTE(Kocho @ Oct 20 2007, 09:31 AM) [snapback]528145[/snapback]</div>

I tried that in less than ideal circumstances yesterday and I am not sure I can make a conclusion. It seems that the noise remains even above 41 mph but then other noises (road noise, wind noise, other traffic) become stronger than it so I am not sure if it disappears or is just overcome by the other noises. As I said, it is not that loud.
I did not get a chance to to that as I did not come across a suitable hill. But when I just shift into N while I'm driving on electric only or on combined gas+ electric at around 38 miles (or any other speed), the noise disappears. But the problem again is that I only hear the noise in the first place if I put a little acceleration thru the electric motor. If I just coast, then I do not hear it...
It very well might be that at that range it becomes more audible, but it actually starts when I drive slow, say 8 mph and sounds like whee-uuu and then becomes more audible and the loudest at about 38 mph or so (may be above that, but as I said other noises drown it thereafter so it is hard to tell). It is there regardless of whether the gas is on or off - only depends on if I press the accelerator to speed-up with the help of the electric motor...
The oil had apparently been chaged at some point, and it was in good condition - pretty good red color and no burnt smell. It would not have been of any use IMO as it was fres enough. Not sure if the few spoonfulls of gunk around the magnet and bottom of the tranny pan would have been of any use, but I did not keep these either.
I think it is not a wheel bearing almost certainly as the noise is clearly correlated to the operation of the car with the help of the electric motor.

Thanks for the suggestions and I will try them again when I drive down hill next time.
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This is interesting and 'less bad.' Had the noise remained at any speed and without "N", I'd have been worried about the burnt coil hypothesis.

Have you tried a "forced charge?" This involves holding the brake with the left foot and flooring the accelerator. The car remains still while the engine revs up to put a charge on the battery. This might give a clue if the noise could be heard. However, this is something that might allow isolation of the noise using a stethoscope. It is not a trivial task and safety measures would be advised to make sure the vehicle can not move. Using "P", which engages a paw in the transaxle might also work along with the parking brake.

Bob Wilson

w4y · Oct 22, 2007

I also have a 2002 Prius that i bought used last week. It makes the same noises you describe. I took it to a mechanic this weekend, and he said he never found anything wrong.

But the more I think about it, the more it seems like the dreaded transaxle humming noise that's caused quite an uproar around here. Nothing is lit up on the dash, however.

bwilson4web · Oct 22, 2007

<div class='quotetop'>QUOTE(w4y @ Oct 21 2007, 11:42 PM) [snapback]528680[/snapback]</div>

I also have a 2002 Prius that i bought used last week. It makes the same noises you describe. I took it to a mechanic this weekend, and he said he never found anything wrong.

But the more I think about it, the more it seems like the dreaded transaxle humming noise that's caused quite an uproar around here. Nothing is lit up on the dash, however.
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Caution, don't project too much. The real hum should be pretty distinct. This is sounding more like inverter noise, which is not a problem. Although, I'll confess I don't hear with my 03 Prius.

Bob Wilson

w4y · Oct 22, 2007

<div class='quotetop'>QUOTE(bwilson4web @ Oct 22 2007, 12:54 AM) [snapback]528689[/snapback]</div>

Caution, don't project too much. The real hum should be pretty distinct. This is sounding more like inverter noise, which is not a problem. Although, I'll confess I don't hear with my 03 Prius.

Bob Wilson
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Bob, it may very well be. I only hear it while braking. Would that just be inverter noise? Is it normal?

patsparks · Oct 22, 2007

I hear a very slight hum from the front of the car when lightly braking going down a hill at 80km/h (50mph), I belive what I hear is normal electrical motor/inverter noise. I only hear it with the radio off, radio fixes it.

hobbit · Oct 22, 2007

My '04 has always made a little "growling" in low-speed heavy regen,
like as speed drops through the teens MPH or so. Given the forces
going on in the transaxle, some of this must be normal ... but if
any of y'all Classic owners can lay hold of a milliohmmeter and see
if your windings are getting out of balance [partially shorted?] or
a high-pot megohmmeter reveals a ground fault, that would be good
to know for the embryonic stats some folks are starting to keep on
this.
.
_H*

bwilson4web · Oct 22, 2007

<div class='quotetop'>QUOTE(w4y @ Oct 22 2007, 01:07 AM) [snapback]528693[/snapback]</div>

Bob, it may very well be. I only hear it while braking. Would that just be inverter noise? Is it normal?
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It isn't something I hear. Just for grins, can you find a nice steep hill to descend at speed and see if it is associated with regeneration by other means?

The real problem is we have to rely on subjective definitions of a noise. It might help to see if you can capture the noise in an MPEG form and post a short audio clip. This would give not only the noise but also the ambient.

Now one trick I've not talked much about is to place a 'disposable microphone' close to where you think the noise is coming from and then try recording. With a little experimentation, you can move the microphone about and eventually find the source.

Earlier this year, I had noticed a 'knocking' sound when using my engine block heater. I used a small tape recorder that I could place on top of the engine to see if I could isolate the sound. It wasn't there! What this means is the 'knocking' sound is coming from somewhere else and I'll have to go 'fishing' for the source.

When the fatal transaxle 'hum' occurs, it should be pretty intense, not just a sound. Placing one's finger tips on the transaxle case should give a real vibration sensation if MG1. For MG2, the wheels would have to be turning, not the sort of thing conducive to putting ones finger tips on the transaxle case. We're talking some pretty strong forces.

As for noises, those pesks can come from so many sources . . . They tell us to be alert but some pretty simple things can cause them.

Bob Wilson

kocho · Oct 23, 2007

Thanks for the replies so far. I managed to record the noise on my PDA. Sorry for the poor recording, but I think if you listen carefully you will hear the noise.

Listen for a, what I think is at least 3KHz frequency whine. It is audible just about from the beginning of the recording, intensifying a bit towards where I say "38". I am gently pressing on the gas pedal to accelerate.

Then it completely disappears around where I say "let go off the gas", since I let go off the gas and started to coast.

For comparison, I also included the noise the regeneration (I think) makes while braking - louder and lower in pitch, when I stepped gently on the brake pedal.

Let me know what you think! The noise does not really bother me too much, just curious what it is and is it normal.

Thanks again!

patsparks · Oct 23, 2007

Sounds just like mine

kocho · Oct 23, 2007

<div class='quotetop'>QUOTE(patsparks @ Oct 23 2007, 09:23 AM) [snapback]529240[/snapback]</div>

Sounds just like mine
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You mean the high-pitched noise as well as the regeneration noise, I hope?

w4y · Oct 23, 2007

<div class='quotetop'>QUOTE(Kocho @ Oct 23 2007, 08:38 AM) [snapback]529244[/snapback]</div>

You mean the high-pitched noise as well as the regeneration noise, I hope?
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Sounds just like mine too. Exactly.

Your voice does too

kocho · Oct 23, 2007

<div class='quotetop'>QUOTE(w4y @ Oct 23 2007, 10:19 AM) [snapback]529257[/snapback]</div>

Sounds just like mine too. Exactly.
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Hope that means it is normal rather than several of us having the same problem

Your voice does too
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That's spooky

bwilson4web · Oct 23, 2007

<div class='quotetop'>QUOTE(Kocho @ Oct 23 2007, 08:05 AM) [snapback]529236[/snapback]</div>

. . .
Let me know what you think! The noise does not really bother me too much, just curious what it is and is it normal.
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Excellent! You might want to look at Audacity, an audio analysis software. I'll try to load the mpeg into my mac later tonight.

I'm going to be the 'odd man out' and say I don't hear that high-pitched tone in my 03 Prius. However, it reminds me of something I have heard:

http://home.hiwaay.net/~bzwilson/prius/motor.html - has an MP4 recording of MG1 current
http://home.hiwaay.net/~bzwilson/prius/magnetic.html

The first URL has an MP4 file that you can play with Quicktime. What is interesting is when the ICE stops, MG1 spins much faster. You can hear the high pitch at the end just as the ICE comes to a halt.

We'll chat some more later.
Bob Wilson

2002 Prius High-Pitched Howl Noise at speed

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