I recently had a similar problem on my 190E 2.3. I solved it by replacing the belt. After the fact I also replaced the tensioner assembly, as a separate issue. I've had the car for only 3,000 of its 140,000 miles, and didn't realize it wasn't supposed to sound "like a diesel" at startup. In my sttemp to locate the source of my "squeek", I also found my "rattle".
A short length of ordinary garden hose works well in a pinch as a substitute for a stethescope.