Help! Water pump pressurizing when using hot water tank.

when opened up for 4 or 5 minutes, hammering goes away. SHut off the last tap and the pump shuts off immediately. Perfect. However - Repeat opening another tap and within a short time - hammering comes back - open everything, let run, hammer goes away. Just noticed that through flex tubing, I can see a persistent stream of pea sized air bubbles on the outlet side of the pump. Not persistent on inlet side. ???