Cavitation can occur when a pump does not receive water under the conditions it needs: low pressure allows vapor bubbles to form and collapse inside the pump. That can cause noise and damage. A rattling pump is not automatically cavitating, though. Treat a new noise, vibration, or drop in flow as a reason to check the water supply and follow the instructions for your exact pump.
Which signs deserve attention?
Unusual rattling, increased vibration, reduced water flow, or changes in pump performance may occur with cavitation. Those signs can also have other causes, including debris, low water at the intake, or a worn component. Record what changed and when it began rather than assuming one symptom identifies the fault.
Check the water reaching the pump
Observe the water level and any accessible intake, basket, or filter for an obvious restriction. Consider whether a valve, pipe, or filter has changed since the pump last ran normally. Turn off power before touching pump equipment or clearing an intake, and follow the manufacturer's instructions. Do not run the pump without the water supply its manual requires while you troubleshoot.
Do not guess at a repair
The right check differs for a submerged pump and an external pump, and the source of a restriction may not be visible. Do not change pump speed, open a sealed motor, or alter plumbing based on a generic checklist. If noise or low flow continues after safe, accessible checks, stop the pump as directed and contact the manufacturer or a qualified pond-pump technician with the model and symptoms.
Related pump checks
A pump that ran without enough water may need a different inspection. See How to Check Whether a Pond Pump Ran Dry. If the issue is reduced flow in a pondless waterfall, Why Is My Pondless Waterfall Flow Low? covers other possible restrictions in that feature. Neither guide can diagnose cavitation from sound alone.