Help! Water pump pressurizing when using hot water tank.

Sigh - air bubbles seem to be a red herring - Old-Biscuit you got me thinking so I removed the strainer completely - no more air bubbles. Thank you. Problem is the 5 sec pressurization problem still persists. HW by pass on, pump shuts off immediately when closing tap; by pass off - takes 5 secs to pressurize. Water hammer on cold inlet pipe to hw tank. On the good side - something clearly not sealing properly on the filter so will replace.