I just went through the same or similar situation with my 60 gallon Dewalt Compressor. All these compressors have a check valve threaded into the tank, it allows air to flow from the compressor pump into the tank, but will not allow it to flow back out of the tank toward the pump. If this check valve gets stuck open the compressed air will move backwards toward the pump and forced through a tube and escape out a port meant to release pressure in the case of a check valve failure. This port IS NOT the HIGH PRESSURE safety release valve that opens in the case the tank pressure becomes unsafe.
Your problem sounds exactly like a check valve failure.
Check valves can fail on their own, but in my case, my year old compressor had a reed valve break which pushed debris into the check valve and caused it to sick open.
I replaced both my Check Valve and then my Valve Plate to resolve my issue.
Hope this helps.
Not to be a PITA but I think what you were hearing was air escaping from the unloader valve. When the compressor shuts down, the main line check closes and the unloader valve opens to de-pressurize the compressor. That way when it starts, it's starting against no pressure. Depending how the unloader valve is configured, it will then shut and the compressor begins operating. If the main line check fails but the unloader valve operates, there will be air discharged from the unloader when the compressor stops until the tank is empty.
