Noise from air conditioning

At a warm outside temperature I notice with my A84,2 after stopping before traffic lights or similar. a noise that seems to come out of the dashboard in the volume of grinding. When starting up, the motot noise when turning up a snoring noise overlaid-noise engine is thus louder. Everything disappears when the air conditioning is switched off.Air conditioning otherwise cools normally and nothing unusual is visible in the engine compartment. There is nothing to be heard at shorter rides and colder temperatures.
The air conditioning is still in delivery in 1995, i.e. Never before has a refrigerant renewal taken place.
What can be the cause of the gereuvhe and how can it be fixed?

Thank you very much

MfG

Gerd