The sound occurs when the radio is on or off, I know for a fact it has nothing to do with the audio system,, i was just using the audio system to give a comparison to the sound i was hearing.,, the reason why i mentioned the motor mount was because in my last mercedes, a similar problem occured and it was the motor mount. I understand that the idle decreasing is normal. I was just giving the forum a play by play of what and when things were happening.. I hear something start up when the car is started, maybe its the airpump,,, but the low frequency vibration is a whole other problem.

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2001 Mercedes-Benz C320 3.2
1991 Mercedes-Benz 300E 3.0