the rubber bushing in the belt tensioner shock absorber shrinks with heat/age. it causes an annoying diesel-like clattering sound and is very common in the m104 engine. the "easy fix" was to use an o-ring as a shim / spacer to snug everything up again.
sorry, that was kind of cryptic wasn't it....
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"