We had what I thought was an air in lines problem, but it was actually that after several months of use and thousands of miles traveled, the pump pressure needed to be slightly adjusted.
The pump was making a machine gun like sound, instantaneously on and off again. What finally made me realize that is was not air, was turning on two faucets on full simultaneously and the noise stopped and the pump ran smoothly. The pressure setting was just barely higher than the flow capacity of one faucet fully open. With one faucet fully on, I adjusted the hex nut until the pump ran continuously.
One other tip, if you have the easy winterizing kit installed and you suddenly start getting a loud knocking when the pump runs. Look and see if the end of the flexible hose is banging on something in there like ours was. I just tucked in behind another line - noise gone.
Eric (and Mary who is in no way responsible for anything stupid I post)
"Beware of false knowledge; it is more dangerous than ignorance." George Bernard Shaw