How do you tell if your air conditioner compressor is working?
Run your engine with the A/C on and watch the end of the compressor where the belt is. If it kicks on you will see the it start turning. if it doesn't turn and never starts turning you may need your A/C charged with freon. If the belt smokes you compressor may be locked up.
The electric fans used to cool the engine are designed to switch on to full speed when the air conditioner compressor is being used. When you turn on the heater/defroster the air conditioning compressor also turns on to help remove moisture from the air. Since both when heating or cooling your car the air conditioner compressor is working the fan will be running at high speed and makes more noise.