2008 Audi 3.2 with auto transmission. Almost 200,000 miles. Engine still sounds great, and the car body is still nice and rust-free.

The car has developed a noise, apparently from the front, somewhat near the center, when driving. Imagine an mostly-empty oil barrel with a little gravel inside. Now imagine that the barrel is mounted on a roller so that you can freely spin it. As the barrel spins, the gravel continuously scrapes along the inside. That's basically the sound (though it is quieter/subtler than my analogy suggests).

The sound appears near the top range of a gear, before the upshift to the next gear. It seems to grow in intensity with load on the engine... so, going up a hill makes it louder. It is the very loudest just before the gear shift, and then it goes away completely with the shift. It reappears when the next gear gets into the upper part of its range.

The sound doesn't happen if I just rev it in park.

I feel like I notice it most in second and third gears, though this may just be because they see the hardest acceleration.

Transmission fluid has likely never been changed. Transmission and engine mounts have likely never been changed. Differential fluid has been changed, but I haven't looked at transfer case yet.

Thoughts on the first place to check?

 

Hoist Review is likely the next step to inspect all driveline components such as brakes, wheel bearings, axles, driveshaft, engine mounts, subframe mounts, etc.

 

It also sounds like detonation to me. Perhaps one or both of your knock sensors is malfunctioning.

 

I suggest dropping the trans oil pan to see the condition of the fluid and amount of sludge.

 

Take your serp belt off, and freely spin all the pulleys, maybe the Tensioner bearing is failing?

 

Thanks for the ideas. I've done a little inspection but really need a better solution to lift the car. Am going to order some of the heavier duty Quickjacks (with extended jack point spread).

Wouldn't detonation also happen if I rev in park? This only happens at higher end of gears.

I've inspected the belt and pulleys for other reasons, and there were no obvious culprits at that time. The noise sounds a little more central and lower than pulleys, anyhow (though I know sounds can be deceiving).

One thing I found was aluminum cups or shields that sort of halfway encircle the front cv shafts, attached to the diff. The one on the passenger side was missing a bolt and was rotated pretty badly. It's possible that it's quiet at lower loads but will do a high speed rattle if the engine is torquing enough. I found what I believe is the correct bolt in my box of hardware that I've removed from Audis over the years (or discovered lying in the garage but couldn't figure out where it fell off). We'll see if that helps. I also found that some of the tinfoil-style heat shielding around the driveshaft and passenger side cv was bent in places. I bent it back.

If one of those things was the problem, I'll count myself lucky. If not, I will probably wait for the Quickjacks before taking a serious look at the transmission. I feel like getting the front or rear up is easy enough, but getting it up and level to work on central parts of the car is an ordeal (and feels dodgy to me).

 

The heat shields were not the issue. Makes sense to me, because the noise happens in such specific gear ranges. Will post if I figure it out.

 

Not necessarily since the engine is not under load when revving in park. Accelerating under load to the higher RPM’s has the ECU listening to the knock sensors for feedback in order to provide maximum power without causing destructive detonation.

If there are engine sensor issues then DTC’s should be present in a scan. A quality tool like VCDS can do advanced logging if you suspect fuelling and timing issues under load.

 

Thanks, I will bear that in mind.

I do have VCDS and recently the only codes I've pulled recently were for the intake manifold tuning valve being stuck open. Those are resolved for the moment, though.

(As a side note: I made those codes disappear in a sort of unconventional way. After testing the manifold flaps, opening and closing them manually, making sure they weren't stuck, and finding no obvious vacuum leaks... I noticed that one of the little vacuum lines attaches to the bottom of the air filter box. Presumably it serves as a pressure limiter, letting a little extra air into the system. Messing around with that seemed to affect the codes... at first I thought not enough air was getting pulled through the nipple on bottom of air box, and it seemed to have some plastic shavings partially blocking it, so i drilled it out a bit... and then I realized that the extra air seemed to be hurting rather than helping... so I blocked that line completely, and the codes went away. I'm guessing that there's a very subtle leak somewhere in the vacuum system, and blocking that line jacks the pressure back up enough to properly operate the tuning valve... but I digress.)

 

Re-reading, I see that you meant to use the advanced logging features rather than just looking for codes with VCDS. I will look into that, thanks.

I realize that I haven't made nearly enough use of VCDS' logging and real-time data.