My 300CD has an inoperative AC system. The compressor is shot and the belt is disconnected. Shortly after I bought it I noticed the same kind of sound.

The compressor had worked loose in its bracket. Made a heck of a racket. I'd check that first. If you idle the engine and shine a bright light down on the AC compressor you'll be able to see if its loose.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.