Although the drive belts are not directly connected to the accelerator, when you accelerate the speed of the engine, you also cause the accessories. (alternator, water pump, power steering pump, and A/C compressor) If the belts are loose, they cannot provide enough friction to produce that acceleration and they slip causing the squeal. Since rubber tends to shrink when it gets hot, the belts, if slightly loose, will tighten after the have slipped a little. The A/C compressor puts a lot of load on the engine. When the A/C is on, the amount of tightening that results from the belts getting hot is not enough to stop them from slipping.
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1966 122s, 1968 142s, 1969 144s, 1979 245dl, 1989 244gl