Refrigerators and most Air Conditioners use what is called a hermetic compressor system. This simply means that the electric drive motor and compressor are sealed inside of a welded steel housing. Inside the housing the motor & compressor unit is mounted on three springs that are used as vibration dampers to reduce the operating noise. When the compressor is running it will normally tilt slightly over on the springs from the torque and when the motor stops the springs will quickly snap the motor&compressor assembly back to the full vertical position. As the units age sometimes one or more of the springs get a bit weak and the reaction is increased resulting in a slight bumping sound when the motor stops. While the slight bumping sound can sometimes be a bit annoying, otherwise there is no cause for alarm as compressors have been known to run for years even with broken mount springs.