Diagnosis
That's water hammer. It's caused by the pressure spike when you release the trigger, combined with a worn or missing pulsation dampener.
How to Fix It
Check the pulsation dampener (a small, cylindrical chamber near the pump outlet). Ensure it's tightly screwed in. If the sound persists, the dampener's internal bladder is likely ruptured and the component needs replacement.
If That Didn't Work
The check valves in the pump head may have debris preventing a clean seal. This requires disassembly of the pump head to clean the valve seats and springs.
Heads Up
Ease the trigger off slowly instead of releasing it suddenly. This reduces the shock to the pump and hoses. Sudden stops are the main cause of dampener failure.