I had a new water tank installed. my house is supplied by a deep well. The check valve clunks 3 times when it shuts off. It is not loose pipes they have all been secured and insulated. It is definatly coming from the check valve. There looks like there is a little knob on the side of the valve. Is this an adjusting knob. The pressure in the tank is perfect according to the manual at 20 psi resting and 40 at shut off.