Air conditioning whistles / howls when the car comes to a stop on VW Golf

Question

I have a problem with my air conditioning in the Golf 7 BJ 11/2014 TDI 2.0 DSG. When the air conditioner is running and I drive up to a traffic light or otherwise stop, the air conditioner starts to "whistle" or "Howl". This happens every 10 or so stops. The duration of the whistle varies from 2 to 15 seconds. If I switch off the air conditioning while whistling, the whistling is immediately gone.

Do you have a guess what it could be?

Answer 1

Too little coolant could be the reason. The whistle comes 99.9% from the expansion valve.