Every 10 to 15 minutes there is a knocking sound coming from the unit. It sounds like a knock on a door.

Robert, as a technician for over 30 years I have run into this complaint several times. I believe the noise you hear is your compressor cycling on and off. When your compressor is shutting off I believe your noise is from the internal compressor valves hitting the interior case of the steel compressor head. Your compressor is a hermetically sealed unit and is welded together at a seem . You have a suction valve and discharge valve that pumps refrigerant through the system. The internal components sit in springs at the bottom of the compressor. It sounds like one of the springs have shifted or busted and the discharge or suction valve is hitting the interior steel head only when the unit shuts off. The only solution is a compressor replacement if in fact this is the noise. Compressors are very expensive and it may be worth $50-60 dollars to verify this is your problem. Matt Ace technician