The difference is where the damage occurs on the impeller.
If it’s too severe case, pitting damage impellers. Let it not be the case, but if that is the case then check where the pitting on the impeller has happened.
Moreover, I asked the operator to throttle the discharge valve, that reduced the flow, and the noise got quiet. If had been the case of recirculation or air entrainment this wouldn’t have happened, the noise getting quiet.
