ICE "Grinding" Noise At Low RPM (2015)

I am hearing a concerning noise from my 2015 Prius which is approaching 130,000 miles. It is a light, metallic, almost grinding noise which originates from the front of the car. It might be pinging from what I've read online, but I'm not very knowledgeable about cars. This is my first "issue" with the Prius. Sorry for the wall of text, but I wanted to provide as much information as possible.

It occurs under these conditions:
1. Low RPM on the ICE, such as when the "Hybrid System Indicator" is just barely past the halfway mark (i.e. you've pressed the pedal just far enough to leave electric mode and activate the ICE).
2. Usually travelling uphill at slow speed, such as when climbing the hill through my apartment parking lot at around 10 MPH. It's not exclusive to going uphill and I've heard it on flat ground before, I think it's just that usually I'm either coasting in electric mode, accelerating, or cruising around 70 MPH on the highway so my ICE does not spend time at that low RPM range in 99.9% of my driving.
3. Once the noise starts, it will continue as long as I maintain around the same RPM. I need to throttle up or drop back into electric to get it to stop.

It does not occur at startup or when idling the ICE, only when under load at low RPM. ICE sounds totally normal under all other conditions.

So far I have ruled out, based on reading past forum threads describing a grinding noise:
1. The hood prop bar is secured, so it's not loose and resonating.
2. There are no rocks between the rotors and heat shields.

Other supporting information:
1. For maybe 6 months (~15k miles) I have been using literally the cheapest gas in town for >90% of my fill ups. It's a Speedway station. Where I used to live, I always filled up at Exxon or Shell. I first noticed this noise maybe 2 months ago. I wonder if all that cheap gas might have lead to carbon building up in the engine?
2. Transmission fluid was drained and filled at 60k miles. Oil changes every 10k.
3. Spark plugs were not changed at 120k as recommended in the service schedule.
4. My commute is 55 miles each way, all highway. I lock the cruise at 70-75 MPH and sit there for 45 minutes. So my Prius has had an "easy life" for 130k.
5. Fuel economy is still good, posting ~47 MPG the last few tanks (calculated at fill up) on winter blend gas in New England; consistent with vehicle history.

What I plan to do next:
1. I'll see if I can get a good recording of the noise off my dash cam tonight so I can share it. I'm not sure if the microphone is sensitive enough to pick it up.
2. I ordered a bottle of "Chevron Techron Complete Fuel System Cleaner" which should be arriving today. I need to fill up tomorrow, so I'll throw some Techron in the tank (and the next few tanks). I know fuel additives are a controversial topic but I figured it's a cheap fix if my issues are due to carbon build up and the product works as claimed.
3. Stop buying the lowest priced gas I can find. Shell is only 10 cents more per gallon and is listed on the Top Tier Gas site ( Homepage ). Again, cheap fix if it works.
4. Change the spark plugs with my next oil change at 130k; I'm going on vacation at the end of the month so I won't be able to get to this until mid-March.

Input is appreciated, again I'm not very knowledgeable about cars but I'm learning as I go with my Prius and slowly warming up to DIY. I'll make sure to update the thread with any new developments. I hope this isn't the start of a serious issue as I was hoping to take this car to 300k miles!

 

Thanks for the replies. I watched a video on cleaning the EGR and I don't think that I'd like to get to that level of disassembly in my apartment parking lot. I can see if I can find a mechanic that will do it.

Here is a video that I took today of the sound. Turned off everything I could, including the ventilation. Sorry for the low volume, I had to turn my headphones up quite a bit to hear it.

The sound starts at 6 seconds into the video, which coincides with me being kicked out of electric mode as I climb the hill.
At 17 seconds, I give it a bit more gas and the sound temporarily stops at a higher RPM.
I immediately ease off the throttle and the sound returns for a couple seconds as the ICE passes through the low RPM range again.
At 21 seconds I re-enter electric mode as I crest the hill and the sound is gone.

Friendly reminder to turn your volume back down.

 

I probably have about 3.5 gallons of regular in the tank, if I fill up with 93 octane I calculate that will put me at just over 91 octane. I assume that's good enough?

I'll probably go fill her up tonight since I have a bit of driving to do this weekend.

 

How's this, Mendel? I amplified the sound quite a bit (500%) using AVS Audio Editor. I can hear it with my headphones at volume 20 instead of 80 like I needed for the first video. Listening to it at the lowest volume you can hear the video at seems optimal for identifying the grinding, since at higher volumes it seems to wash into the wind/tire noise.

 

Interesting results. I topped up my tank with 8.5 gallons of Shell V Power Nitro+ which said it was 93 octane.

I drove around town for a bit after filling up and it was grinding/pinging more than usual. I usually don't hear it around town, only on my hill. Found some nice empty roads and drove like a dingus to burn fuel and clear out the 87 octane gas from the fuel lines (is that even a thing?).

Put 20 miles on her, and about halfway through my drive I started noticing that the ICE was still grinding/pinging when passing through the low RPM zone but I couldn't keep the noise going like in the video. It would do it for a half second then stop even if I maintained the same RPM. Went up the same hill 3 times in a row and I couldn't sustain the noise like I have been able to every day on my way home recently. So it's... slightly better?

Random note in case this is outside temperature related, it was over 10 degrees colder for this drive than in the video (35 F vs 47 F).

 

A couple updates,

Firstly I ran through the tank of Shell V Power Nitro+. The main thing I was impressed with was how much better my Prius felt and sounded compared to the past few months. I can't explain it exactly, just that it sounds "like new" again, especially when giving it some gas to get up to speed on the highway. Acceleration feels smoother. Still getting the noise at low RPM going uphill, but it's still as described in my prior post where it will make the sound for a half second and then stop. I really don't think the octane helped me here, rather the additives in the gas. And maybe the fact that the cheapo gas I was buying for half a year had contaminants and/or water in it.

I received my Chevron Techron this weekend. Yesterday I filled up with 87 octane at Shell after putting in 11 oz of Techron. On this morning's commute my car felt even smoother than with the V Power. It was 25 F out this morning and I'm used to my car sounding like she's struggling a bit more on cold days like this. Impressive MPG of 47 (HUD reported) on a 55 mile trip, which I would have never expected to see given the outside temperature and high headwinds gusting as high as 40 MPH. I would have expected the HUD to report in the low 40's based on past experience with similar conditions.

After taking a look at my Fuelly history I retract the statement in my original post about MPG being consistent over the vehicle history. There's a noticeable downward trend over time. We'll see how the rest of this tank goes but I'm thinking it might turn me into a believer in fuel additives.

 

Yes, Techron is reported to have PEA in it although the bottle only lists it as "proprietary additive". Those who believe that fuel additives work seem to mostly agree that ones with PEA work the best.

Yes, the Shell V Power was 93 octane, which resulted in a blend of roughly 91 octane when being mixed in with the Speedway 87 still in my tank. I think the high octane test is to see if significant carbon deposits in the engine have reduced the volume, thus increasing compression ratio and causing lower octane fuel to ping? Regardless given my 3 data points (Speedway 87, Shell V Power and Shell 87), my conclusion is that the higher octane did nothing and any differences observed are due to overall gasoline quality and additives.

I was using HUD to refer to the "HSI display" or whatever it's called, since I don't know the proper name. I wasn't aware that any Prii offered an actual HUD, but now it makes me wish I sprang for the Four/Five instead of the Persona.

 

Also, after doing more research I wonder if I might have misdiagnosed my issue. I read past issues about rocks in rotor heat shields causing a grind/rattle, those checked out good on my Prius and I moved on from heat shields. I'm now aware that cars have exhaust heat shields as well. This video sounds exactly the same as the sound I hear in my Prius:

It sounds like the exhaust manifold heat shield may be my culprit? As the noise definitely seems to be coming from the engine area. I want to check it out on my car, unfortunately it's bitterly cold here today so it will probably have to wait until the second weekend in March when I come back from vacation. Nice to have some hope that there may not be an issue with my drivetrain just yet.

 

Been a little busy lately.

Got the spark plugs replaced a couple weeks ago. I described the sound, how to replicate it, and that I thought it was the exhaust manifold heat shield rattling. He test drove my Prius twice and said he didn't hear that noise or any other noises of concern. Of course on my way home from work the next day I got it going up my hill! So no dice on the spark plugs, I think that helps prove that this is not an issue with the ICE. I'll just learn to live with it unless it gets worse.

I now have a NEW sound though, a nice creaking when turning the steering wheel at low speed.

 

The sound has reduced, but I think it's more a factor of the outside temperature. In the colder weather I noticed the sound got louder on days where it was in the 40's than on days where it was below freezing. As it warmed up past that point into the summer, I have a hard time re-creating the sound and when I do it's much quieter (I think I barely heard it one day after work last week when heading up my hill). For me this is more evidence that it's a rattling heat shield somewhere, and that there's some sort of thermal expansion thing going on. We'll have to wait until next winter to see, but for now I'm not too concerned.

The creaking steering wheel sound is still there, but again much reduced in volume over time and I also attribute this to the seasonal change.

I will say that the fuel additive was well worth it. Acceleration and fuel economy both improved in very short order and the car just sounds "healthier" when the ICE is running if that makes sense. I only did the one tank of Shell V Power+ and the one tank with the Techron that I posted about earlier. Since then I have used nothing but "Top Tier" gasoline in my car (mainly Shell 87 octane) and I have not noticed any signs that I would need another dose of Techron.

My Prius is now approaching 140k miles (+10k since I started the thread) and I've had no new issues. It's at the point where I'm expecting something to break any day now just due to the sheer mileage that I'm putting on this car but I'm always pleasantly surprised when I get to that next oil change interval and everything is still running smoothly.