Estonia

Hi. I had extremely loud knocking in engine - video is here
What could cause such a noise when idling?

2004 A8L 4.2 Quattro, engine model BFM, mileage 345 000 km. No engine fault codes.

Thank you.

MP4.2+6.0

Offhand it sounds like it could be a bad connecting rod bearing, in which case the engine is basically gone. At 200K miles/345K km, not too surprising. You don't get codes for many mechanical failures, especially core internal engine like this could turn out to be.

Estonia

This was also my initial diagnosis but whilst shopping for a new engine my mechanic decided to remove the passenger side camshaft cover and found that while turning the engine over by hand the chain tensioner made strange popping metal sound (later we found out that this could be normal behavior). The sound is here
I had replaced both tensioners with Audi originals few years ago but they were out of warranty because replacement was not made by Audi – they wanted to drop the engine but it can be done without that.


After giving it some thought I bought a non-original tensioner and after installing it the noise was gone.

Everything went back together and after few starts the noise was back - it was very mild but still, the same noise, metal on metal. Clearly changing with engine speed and constantly there.

So, everything came back apart and a new original chain tensioner went in. Everything went back together again (actually they did it twice because they managed to mess up the timing but did not test it before I arrived to drive away) and the noise remained.

It was mild but clearly metal noise, for about 15 seconds, always at cold start (it was spring so 5 degrees Celsius and up for us here), sometimes also at warm start. Speeding the engine did not change the time it took to go away. Decided to just drive it to its end but the sound went completely away in about a month and few hundred kilometers.

The reason I’m writing is that after a year and 5000 km it’s back again, mild, but the same noise.

Does that change your opinion?

Thank you.

Estonia

Sorry for the typo, correct video for last post is here:

MP4.2+6.0

If you don't think it is con rod bearings, only other thing I can think of is to check/replace are the anti drainback valves in the cylinder heads. They are buried under the access plate in the valley of the "V" under the intake manifold, or at least were on my C5 4.2. A forgotten part on many Audi motors. If you want to go quick and dirty with old and maybe tired engine anyway, you could try an oil solvent to see if that will clean related passages--oiling of head, lifters, rocker shafts, etc. Rislone or Gunk would be USA examples. Basically light petroleum solvents. Pour in. idle engine for 30 minutes or so, then change oil and filter. Obviously better near an oil change time cost wise.

Estonia

Hi, I changed those valves 3 years ago in a different place that rebuilds engines. I will check them before swapping the engine, thank you.