Strange Knocking sound from piping at certain times?

CPM10V

Active member
Jun 12, 2019
29
Phoenix, AZ
I’ve noticed that when people are playing in pool (mainly jumping around, resulting in a “wave” action in the water) occasionally I’ll hear a knocking/thudding sound that appears to emanate near the skimmer & portion of plumbing that is above ground, you can feel pipes vibrate slightly. I turn pump off as soon as I hear it & wait a few minutes, start back up & it’s gone. Does the waviness of water going in single skimmer doing this? Some sort of air lock? Any way to prevent from obviously not jumping around? Thanks!

1976 in ground, 15 x 36, diving end, plaster, single skimmer
 

CPM10V

Active member
Jun 12, 2019
29
Phoenix, AZ
It started it again this evening actually, heard the knocking sound & no one was in pool other than the robot. I need to top off water level it’s probably a bit under the normal midway point. I’ve got one of those diverter valves in bottom of skimmer so if water was low wouldn’t it pull from main drain? Or does the pump just need one instant of water level below weir door to pull air? Still learning, thx for the advice.