I recently replaced a A.O. Smith 1 HP C48K2PA105 motor with a A.O. Smith 1 HP Square Flange 48Y Up Rate EE Motor - UQC1102. It is mated to a Sta-Rite Dura-Glas P2RA5E-124L that contains a C105-138PEB impeller and a C1-200PA diffuser. At low pressures (i.e. 12 psi after backwashing) there is a noise in the Hayward multiport valve that sounds like stones are rattling around. At higher filter pressures (18 psi), the noise goes away. I think that I am experiencing impeller cavitation.
Is the replacement pump that I selected the correct pump?
If so, since I can't reduce the pump speed to eliminate the cavitation, can I change to a smaller impeller/diffuser to eliminate the cavitation, perhaps to a C105-92PS/C1-216P?