There are a couple of things that could cause this: 1. The compressor may be running to pump the tank up to set the cut-out air pressure rating. 2. The compressor controls may be set for continuous run operation where the compressor will unload but not shut down. Change the switch control for a start-stop operation cycle. 3. The pressure switch may be bad and needs to be replaced, repaired or adjusted. 4. There may be air system leaks that are creating an artificial demand for air and the air compressor is trying to overcome the air pressure loss caused by leaks.