Sometimes the engine belt idler pully can develop a problem. I don't know how these terms translate to your version of English so I'll try to describe them.

"Engine Belt" - The rubber ribbed belt that drives the things on the front of the engine, like the air conditioner compressor, the power steering pump, the fan, etc.

"Idler Pully" - This is the round pully that applies pressure to the engine belt to keep it firmly on the engine. It is spring loaded and just holds tightly against the pully. I believe it may be circled in red on an illustration above.

The idler pully bearings can wear and cause problems. The clue is the noise when the air conditioner is turned on. Go look at the engine belt when the air conditioner is turned on - if the belt is jumping around a bit then this is the location of your problem.

Best to resolve this problem before the idler pully actually fails because the belt can cause damage as it comes loose from the engine.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles