I have an inground pool. 2 inch plumbing throughout, except necked
down to 1 1/2 right before the intake to the pump. The output goes
through a filter system which has pressure gauges and then to the pool
return line (2 inch) and the spa return (also 2 inch). The pump is
putting out 1 1/2 to 2 hp. When the pump runs, it sounds as if there
are bubbles in the intake line. NOT the pump itself, but the intake
line leading to it. I don't see any air in the pump or lines. When I
close off 1/2 of thr output by closing down the pool return, leaving
only the spa line open, the presssure in the filter line jumps from 15
psi to 22 psi and the noise in the intake instantly stops. Open the
pool return line again, and the noise instantly returns. Is this
cavitation? The motor seems to run hot, but has had no problem putting
out flow or tripping breakers. What can I do? I can't reasonably
change the intake to 2 inch without spending big $$ as all the valves
will need to be changed.

Is this cavitation or an air leak. I've never seen cavitation before ,
and know little, but it sure seems like it to me.