2002 Clk 320

Hi,
My car vibrates a little when I'm sitting in idle. I've taken it to two different dealerships and they claim that their diagnostic test do not show any problems. However, the vibrating is very noticeable to myself and anyone who rides in the passenger side.
One of the dealerships even replaced the motor mounts, but said that they weren't the problem.

THEN WHAT IS CAUSING THE PROBLEM??!!! Is anyone else having this same problem??

Thanks