Cavitation

Cavitation sounds like gravel or marbles in the pump. It happens when the pump cannot get enough water. Tiny vapor bubbles form and collapse, and they pit the impeller over time.

Causes include a clogged basket or skimmer, a closed valve, a low water level, an undersized suction line, or an air leak. Shut the pump off and check baskets and valves. If the sound continues, have a pro inspect the pump, since ongoing cavitation shortens motor life.