Mercedes 380 SL knocking

It sure sounds like a stuck lifter. I cannot understand why it goes away when you speed the engine up. The first thing I'd do is go to my local auto store and get an oil additive to free up stuck lifters. YMMV. If it doesn't work, you are not out a lot of money.

Let us know what the solution is. Good luck.

EJ > If anyone knows, what could be the source?

If I had to guess, it is NOT a lifter NOR the cat. It does remind me of a cracked flex plate (flywheel). Does it change any from cold to hot? Does it still do it at highway speeds? Any other clues?

Metallic rattle sounds like the catalytic converter's honeycomb is loose or broken.

This cat converter sound is easily checked by hitting with a hammer.

Stuck lifter sound can be drastically reduced by switching to synthetic oil. It will sound alot worse in beginning, but after driving around town, it will quieten up.

The knock is one per revolution and goes away when engine is accellerated in speed. It comes again at a specific point near idle when deaccellerating the engine speed.

A stuck lifter would to my opinion not go away at higher revolutions that way... just the contrary, and the sound would not be that sharp.

It sound more like something with engine suspension or anything on or near the engine (with some mass) being loose. At low rpm's the engines displacecement due to vibration will be higher, and at higher rpm's whatever is loose will not be able to follow the vibration.

I once had exactly same symptom. It turned out to be the shock absorber for the belt tightener, which had lost its rubber bushing, causing the bolt to knock at the perimeter of the shock absorber fixing hole - only at low rpm's. The sound was difficult to locate, and I too spend a lot of time looking for at stuck lifter.

So, look for something simple.

On May 24, 6:38 pm, "-->> T.G. Lambach

On May 24, 6:38 pm, "-->> T.G. Lambach

Point well taken, will research some more. Thanks

Will try both . thanks, Alex