My 91 300TE with the M103 has developed a knocking noise at idle. I'm not sure what it is but I'm wondering if its possibly a sticking lifter. The noise goes away if you rev the engine and its perfectly quiet and smooth once its warmed up. I'm attaching videos of a cold and a warm idle below.


Cold engine.
https://www.youtube.com/watch?v=f7sTo6eUQKo


Warm engine. This is after a 15 minute drive.

https://www.youtube.com/watch?v=Ff1oM05i3Eg