Window Air-Conditioner unit rattles loudly when it turns off?

The compressor in all window air conditioners is a powerful motor. When it starts up - and especially when it shuts off - the whole air conditioner can shake, sometimes loudly. Usually there is nothing that can be done to correct this problem. However, it is possible that the compressor mounting pads and brackets are worn out or missing. If that is the case the pads and brackets can sometimes be ordered and replaced.

Be sure that the air conditioner is securely mounted in the window frame if the unit shakes at all. It is possible for the unit to shake free of the frame if not installed properly.