2007 Prius Whining Noise From Engine Compartment (Proportional To Acceleration)

Hi guys,

I just created an account here and conducted some searches through past posts in the forum to attempt to diagnose an issue I've had with my 2007 Prius with 79,000 miles on it. Several months ago I noticed a distinctive whining/humming noise coming somewhere from the engine compartment of the car while driving. The noise may be present at lower speeds, but the volume of the noise appears to increase proportionately with my acceleration and as such is most noticeable at highway speeds of 50 - 70 miles per hour. As I accelerate faster, the noise level seems to increase. If I take my foot off the gas and coast, decreasing speed, the whining noise gets lower in volume. I believe I first noticed this noise about 6 months ago, but wasn't more proactive about it like I normally would be because at first the noise didn't seem overly loud and my extended warranty unfortunately was up once I hit 77,000 miles. Now 6 months after I first noticed the noise, I believe it's definitely louder than it was when I first heard it, so much so that I can pick it out over the radio being on at a decent volume.

After checking some forums I thought the culprit might be a wheel bearing. I took it into a local Midas shop at they informed me that my passenger-side front wheel bearing was actually breaking apart in pieces. Obviously I had that wheel bearing replaced but I was also hoping this was the source of the whining noise. Unfortunately that wasn't the case as driving it home the noise was still present. They also said the tires were not balanced properly (from my Toyota Dealership no less when I had them install new tires) so I had the tires rebalanced as well. They did take the car for a test drive but probably stop looking for issues with the car as soon as they saw the wheel bearing in that condition. Also that shop does not specialize in Prius' or hybrids so they might not be able to notice the noise that shouldn't be there like I can.

Other than wheel bearings does anyone else have any ideas of where this noise could be coming from so that I can have some idea of what I might be in for (expense-wise?). Some posts on here possibly pointed to the ICE water pump or transaxle fluid (which as far as I know has never been replaced since I bought the car new). I know I'm not crazy and this is a real sound with an issue behind it... I can't help but focus on this noise every time I drive my Prius and I loved that car because it was so quiet.

 

I don't *think* it's the tires. The treads still look good and the tires aren't balding because I haven't put a whole lot of mileage on the car since I had all 4 replaced in June of 2013.

Also I my model Prius doesn't even have a USB Port, just an Auxillary port so I don't think it's any noise coming through the audio system. While I'm driving it distinctly sounds like it's "outside" the car by the engine rather than coming from inside the car. I wish I could describe the noise better... an electric whine? A whirling noise like a propeller blade? Something along those lines.

If a belt is damaged in some way (like the serpentine belt) would any rubbing or friction be able to produce a sound like that??

 

Thanks for the ideas guys. I'm going to try to get a better idea of where exactly the noise is coming from and at what RPMs hopefully tomorrow or Thursday when I can enlist the help of my g/f hitting the accelerator with the car in park while I check under the hood.

When I had initially brought the car in to Midas they checked both front wheel wheel bearings; the one that was falling apart was replaced, the other (driver's-side front) was in good shape. Unless the noise is actually coming from a rear wheel bearing (which it doesn't sound at all like it), I think I can eliminate the wheel bearings as a cause (I don't notice a change in the sound when I turn the wheel sharply left or right while driving as well). I will need to look back in my records to see if my transaxel fluid was ever changed. I did some dealer "package" maintenances over the years (filters changes, oil, tires...etc) so they may have changed the transaxel fluid at that time but I need to look back in my records. If not then obviously I am well overdue for changing that fluid.

 

So after finally taking a really close listen to the car with the hood open and the accelerator being depressed, there is definitely some type of "fluttering" noise that is audible even without the car in motion, so that to me rules out the tires. I can only hear that noise when the gas engine is active (not when it's only on battery) and it sounds like it's directly under or on the side of the gas engine portion of the engine compartment. To me it sounds like something is rubbing/some kind of friction, possibly a belt? All the fluids are at their full levels where they should be inside the engine compartment. I'm going to take it to my local Toyota dealer in two weeks and I'm pretty sure they would be able to diagnose it right away because it's audible immediately. It only gets louder as the car is in motion. Hopefully it's nothing too expensive.

 

So on Monday I finally had a chance to bring my Prius in to my local dealer that I've had all my car's major service performed at. Unfortunately I was disappointed with the outcome of what they found regarding my noise: absolutely nothing They said they road-tested my Prius (it looks like only for four miles), checked all the belts, tires and everything else they could think of checking that might have been the culprit and found no abnormal noises! I couldn't believe it and had them re-check it only to have them come back with the same results. To be completely honest I would've rather they came back with bad news that could be fixed than no news at all. This makes me think somehow I'm hearing a noise that no one else can hear... but when I have to raise the volume on my radio to drown out the sound of my car's driving, I would like to think I'm not crazy... but how could trained Toyota mechanics not hear a noise I feel like I can hear without the car even needing to be in motion? I would've thought for sure they would've loved to find the cause of the noise and have me pay for yet another fix or maintenance (I had air/cabin filters, throttle body service and fuel injection service done while I was there). I wish I had two Prius' so I could drive them both at the same speeds and listen for any discernible difference in noises.

I'm at a loss of what to do now... do I live with what I feel like is noise caused by something? Do I bring my Prius to a different dealership? I'm not a car guy... I'm very handy when it comes to electrical, computers, small jobs around the house... but I'm not a car guy. I was relying on these mechanics to find something. If they say there's no issue I really didn't have much of a Plan B.

@ Pulsbeat: What were the results of your visit to your dealer? Did they actually come up with an answer for your Prius' noise??

 

I took a video while I was driving which actually does a pretty good job of making the noise I tried describing stand out. I was operating my Prius at highway speeds (55 - 65 mph) with the windows closed. I constantly hear this vibrating noise when I drive. Maybe some of you can listen to this clip and validate whether or not the noise is normal, and maybe even help me to identify what it is. As I mentioned in my previous post the Toyota dealer I took my Prius to told me they could not hear any abnormal noise or anything wrong with my car. Thanks for the help... this noise is driving me nuts :/