Diagnosis
That's pump cavitation. It means the pump is not getting enough water. The inlet water supply is restricted.
How to Fix It
Ensure your garden hose is fully turned on. Check that the inlet water filter screen (inside the unit's water inlet) is not clogged. Remove it and rinse it clean. Make sure you're not using a hose longer than 50 feet or one with a small diameter.
If That Didn't Work
The check valve inside the water inlet might be stuck. You can try to free it by tapping the inlet fitting lightly or removing it to inspect.
Heads Up
Continuous cavitation destroys the pump internals. If you hear this sound, release the trigger immediately and find the water supply issue before continuing.