I have a rattling noise coming from the engine, not necessarily on the cold start. During operation, particularly at idle something rattles. The tech said that it is a timing chain and there is an easy fix ( $200 ). He will replace a chain pensioner or something like that, which can stretch further than the original and keep the chain tensioned. Is this possible ? Car is a 300CE 24 with 85,000 mls.

Rattling noise in 300e
 

Not sure if this helps, but my 300E had a rattling noise in which a mechanic diagnosed as being a timing chain, turned out to be the motor mount shock absorber. Try holding something against it while you hear the rattling.
Joel

I agree with Joel. Try the easy stuff first. BTW, it is the little shock in the front inmediately behind the belt, to the right side of the engine (looking from the front). PartsShop should have it.