400SE AC drone

Hello:

This is my first time posting to this list. Here's the problem.

Car: 1992 400SE 103K by the book 100% complete MB maint.

When the climate control is on there is a very noticable droning/humming sound at exactly 1100 rpm. It happens at idle, and most noticably on deceleration. I have isolated the noise in many tests to only occuring when the CC system is activated. I was told by the service advisor that the compressor could not be bad [as I suspect] as it was activated by a magnetic clutch that is activated by temp sensors in the car. So, If the car was cold inside and you turned the dials both to "High" the compressor should not be on as the temp being asked for by the control unit would be far below the temp inside the car, but, the drone is still present.

I have have had the serpantine belt and tensioners replaced by the dealer which of course did nothing. I then had the frame rail bushings that somehow buffers contact with the cc system replaced by the dealer. The said the problem was a resonance on the frame because of a worn bushing
but again, that has done nothing.

The car was Starmarked [which of course has run out]when the problem was first reported and If I can get an accurate diagnosis, I'm going to try and make them pay for the fix. Any information would be very helpful.

TIA