Shurflo Onboard Water Pump

[McBoaty]

Many thanks for your reply. My installation is on a boat & I've noticed that the knocking noise stops once we've used the taps after having travelled a bit. Your explanation of hot water expansion may be the answer since using the engine heats up our water tank (which well lagged can stay hot/warm for a considerable time) which may be why the knocking seems to go away again after frequent use and reappears once again between trips & it's been standing (cold)again. I have checked for leaks (couldn't find any) but since it's an intermittent/temporary problem had tended to rule that out too. Not sure why the old pump didn't suffer from same issue but it may be the slight difference in flow/pressure ratings tipping the balance....I mayy try tweaking the pressure switch slightly (easy on Shurflo) setting but now also reassured by a reasonable explanation...much appreciated...