Another thing you may want to inspect is the belt tensioner shock. My old 300E made a sound that sounded like chain slap at initial startup and low idle conditions. Against the dealers advice at 200,000 miles I decided to have the chain and tensioner replaced. The noise remained.
It turned out that the bushings at either end of the shock had hardened and shrunk causing the shock to rattle on the bushings. The noise only occured at low engine speeds.