I am changing the belt, pulleys, and tensioner on a preventive basis, so that will provide some indication if it was one of the pulleys or the tensioner. The sound is too hollow and low pitched for that, in my opinion.

I'm now seriously thinking it's something rattling around on the exhaust.

The rattle is only
1) When warm, and
2) At or near idle speed (600-800 rpm).

Note the rattle is not quite regular. It seems to me a piston or bearing would be very rhythmic. There is a steady ticking audible with the stethoscope on the underside, but I don't know whether that is the same noise or normal.

Thanks for the thoughts. Every different perspective helps.

 

I listened to the audio files again. That sound is on, or in the engine. I agree it doesn't sound like the belt or pulleys, but those are easier and cheaper to replace. I am always a fan of throwing parts at cars, because the worst thing that happens is you have new parts in the car. Best thing that happens is you solve it.

After the belt and pulley replacement, if you can post another warm idle audio file, that would be appreciated.

If you are thinking exhaust, it might be a heat shield, loose flange or cracked/loose manifold attachment to the engine. I think this sound is coming from in or very near the engine. Exhaust things like these aren't posted about on this site too often.

Exactly where are you holding the microphone when you record the audio? How close to the engine is the microphone?

 

Did the pulleys & belt. Holy cow what a pain.

No improvement to the noise, and I can't help but think the operation was a waste of time, skin, and money. It was hard to tell the old belt from the new, and all three pulleys seemed fine. Engine seems quieter, but that's probably wishful drinking.

The mic was on the garage floor, on a towel, and under the front passenger door.

I'll try stethoscoping a bit more. My road trip is coming up fast, so I have to cut the investigation short pretty soon.

Thanks for the time and attention.

 

Eric, did you make any further progress on this? My 07 GL450 with 165k miles makes almost the exact same noise when warm, but runs incredibly well and is very smooth. Kind of has me stumped. Did you every try adding gear oil??

 

I did not make any further progress. I got overwhelmed by moving (thank gawd that's over) and the truck purred like a kitten on the 900 miles to the Free States. I will turn my attention back to it after getting a house in which I can get back to wrenching. Here's to hoping it does not force my attention sooner.

I did not tinker with adding gear oil. Here is a word of warning regarding that topic. The oil pump does not push the oil through one long tube. Rather it is many short tubes in parallel. This is significant because the oil flow is, or should be, tuned for a certain viscosity - and this is important because the tubes are not all the same diameter nor the same length. If the viscosity is too high, the longer and narrower tubes will get proportionately less flow. This is one reason oil coolers are one tube snaking back and forth while water coolers are a many tubes in parallel. If you ran oil through a radiator, the oil that had cooled in one passage would thicken and the hotter tubes would pass more oil, defeating your attempts to get rid of heat.

So you are really taking chances by fiddling with the viscosity.

For a while I ran 10W-30, assuming the mild climate of San Diego indicated the expanded range was unnecessary. It turns out the viscosity of the two oils is significantly different (13.5 vs 10 cSt) at operating temperature (100C). However, the motor seems pretty happy overall at 170K miles, so I suppose I didn't do too much damage.

This also implies that it really isn't a good idea to run the motor at full torque (open throttle at 3k rpm) when the oil is not fully warm.

Of course, leaking oil will be alleviated by adding higher viscosity oil. I happened to have a couple of bottles of STP oil treatment, which I was going to try adding to help the leaky valve stem seal, and it appears to have the viscosity of gear oil. I mean really heavy stuff; heavier than the 75W-90 that I put in the differentials. Are they claiming the stuff stops leaks when all it does is goose the viscosity? Now I'm nervous about adding it to the motor, especially as we're going in to winter.