My 83 240D with AC Compressor on shakes like crazy at idle. It sounds as if the compressor is loose or a cheapo ( I got it from Pep Boys) but it really does pump cold air! The compressor is on tight, of course. Its just that when I'm at a stop light, it sounds as if something will fall out of the engine compartment.
I use the same compressor on my 85 300D and its very quiet!!


83 240D with AC Compressor on shakes like crazy at idle

carrameow,

With the engine NOT RUNNING, reach in and turn the very front plate of the compressor clutch with your hand. If it does not turn smoothly with no metallic feel to it, you should consider replacing that compressor before it fails internally and spreads metallic debris throughout the system.

Then, as you have already determined, loose mounting could be causing noise, but I don't think it would effect idle quality.

Best of luck,