Irritating water system/pump problem

Got a very irritating problem with my water system/pump, in that it will not stop running entirely but when on, keeps giving a “one tick/pump/echo clunk around the van”! I have tried running all hot and cold taps for quite some time and making sure they are securely shut after use, so reasonably sure there is no air in the system, or is there?

The motorhome is a 2001 Rollerteam Auto Roller 70 and the pump is a Fiamma Superflow 126 PRO, which otherwise works fine i.e. it delivers water on demand and there are no water leaks or cracks in the body that I can see.

 Any ideas, anybody?