Knock, knock. Who's there?

[moribundman]

This is an Audi 2.8 liter V6 engine (not mine) with 134k miles on the clock:

A knocking sound started showing up a few months ago. The knock is pretty loud and it only seems to knock at idle. However, the 2.8 is very loud so it may still be knocking but you just can't hear it above 2000 rpm. The knock seems to be coming from the left bank of the engine and it seems to be knocking at crank speed. The engine will not knock if it is cold, it will only knock when the engine is at or near operating temperature.

When the engine is up to temperature and when the car is facing upwards on a slope of about 20° or more, the engine will not knock. However, when descending that slope the knock will start to come back as the car approaches level ground, and the knock will get even louder if car is facing down on a slope.

I was thinking rod or crank bearing issues, maybe excessive shaft endplay. I told the guy to rub some oil from the dipstick between his fingers to see if there are any metal particles that hint at bearing damage.

Any suggestions?

 

[jldcol]

Motor mount. You can make a reasonable substitute out of the cheesy stews you like so much, or even one of your cheesy answers, or even this one. The goal of the universe is to end, or continue goal being the end of time. Natural selection is the thing working on the bigger, better idiots tis working oh so well so far. IIRC Tata Nano was always ready.

 

[aaxb970]

Motor mount. +1

 

[moribundman]

I wouldn't discount a defective engine mount. Didn't seem likely to me, because the A4's fluid-filled engine mounts very rarely fail. The front-mounted torque rod however does fail after about 6 or 7 years when the rubber bushing splits. That causes a noticeable "clonk" when shifting. I'll have him check the mounts and the torque rod.


My Buick and Chevy had nonstop issues with the engine mounts (and everything else), but it never caused knocking sounds at idle. The "dog bone" mount on the front (if I remember correctly) broke on both cars a few times.

 

[JTK]

I've seen weak timing belt tensioners slacken the T-belt to the point the T-belt slaps against the inside of the T-belt cover. It sounds disturbingly like a rod knock. This noise typically goes away at RPMs above idle. Might be somethin' to consider.

Joel

 

[mechtech2]

That 20 deg thing stumps me. Maybe the guys are right about that motor mount.

Tensioners and pulleys are possible - either the serpentine belt or the timing belt. And of course, a rod or main bearing may be puked.
Someone will have to dive in and check further.

 

[moribundman]

JTK and mechtech2, a belt tensioner seems like it might be a possible cause, though I haven't heard of it happening on that engine. When the TB tensioner goes bad, failure is usually immediate and catastrophic. Serp belt tensioner failures are uncommon on that engine. I'll pass on the recommendation of checking out the tensioners and pulleys.

 

[gathermewool]

When you're ascending, the engine is under load. While descending, the engine is under only compression loads as the injectors are most likely not injecting.

Could it possibly be piston slapping. Under a low load, the piston head may be "slapping" against the valve head very slightly. I'm not familiar with Audi, but my 97 Legacy has this very common issue. Heck, a TSB was put out while the car was still new attributing this piston slapping to an engine redesign, and that it is entirely normal.

 

[moribundman]

If it's piston slap, then it's not normal for this engine and something is seriously wrong.

 

[The Critic]

Originally Posted By: JTK
I've seen weak timing belt tensioners slacken the T-belt to the point the T-belt slaps against the inside of the T-belt cover. It sounds disturbingly like a rod knock. This noise typically goes away at RPMs above idle. Might be somethin' to consider.

Joel

Yeah, I saw this one firsthand today. It was an older KIA Sedona and it sounded like a rod knock, but the timing belt seemed to be a bit on the loose side.