Diagnosis
The pump is cavitating. This means it's sucking air, not water. Air in the pump destroys internal components fast.
How to Fix It
Immediately release the trigger and turn off the machine. Check all hose connections for tightness. Ensure the garden hose water supply is fully on. Start the machine with the trigger pulled to purge air. The noise should stop once water flows steadily.
If That Didn't Work
The inlet valve or internal O-rings in the pump may be damaged from running dry. This requires pump repair or replacement.
Heads Up
Never start the pressure washer without water supply connected and flowing. Running it dry for even a few seconds can wreck the pump seals.