Diagnosis
The pump is cavitating. This is almost always caused by a restriction on the water inlet side, not getting enough water.
How to Fix It
First, make sure your garden hose is fully turned on. Second, remove and inspect the inlet water filter screen inside the quick-connect hose fitting on the machine. Clean any debris off the screen.
If That Didn't Work
The unloader valve may be stuck open, bypassing all pressure. With the machine off and pressure released, locate the unloader valve (brass fitting where the high-pressure hose connects). Tap it gently with a plastic mallet to free it.
Heads Up
Always use a pressure washer with the garden hose tap fully open. Running it with low inlet flow will damage the pump seals from overheating in under a minute.