Belt tensioner?
 

From a cold start, something in my engine compartment will rattle for the first 2,000 rpm of accleration. It shuts up after 2,000 or so RPM or when the car is warm.

I took a peek at my belt tensioner pulley and it is indeed at an approximate 2 degree angle off from the crankshaft pulley.. time for replacement? How much of a PITA is it?

belt tensioner puley
 

Not too difficult. It helps to remove the belt, not entirely, but to relieve pressure while working. P.

How does that pulley come off. It appears to have a cap or something over the end on my E300D.

The cap snaps off with a screw driver under neath is an allen screw. The new pulley comes with a new cap. I just replaced my pulley just in time before it self destructed.

I have put a new belt on and noticed some of the surface on the smooth side coming off. The belt seems ok otherwise but I wondered if the pulley had some roughness on it causing this. I checked pulley and it was smooth and tight when I replaced the belt, so I figured it was just natural for the new belt to shed the thin layer of rubber over the kevlar belt. My old belt was smooth on the back when I took it off. Also, since the belt comes off the pulley and the back sides are moving opposite to each other and are very close together, I wondered if slack was causing both sides to rub? I can see no evidence of this and the tensioner shock seems good. the debris is fine rubber particles and easily rubs off the back. I hope I am making sense in this description. Anyhow, have decided that if it continues, that I will have to consider changing out the shock, and pulley. What do you think?