Diagnosis
Air in the pump or a leaking inlet connection. The pump is drawing in air along with the water.
How to Fix It
Check all inlet connections. Ensure the garden hose is tightly connected to the spigot and to the pressure washer inlet. Make sure the garden hose itself has no leaks. With the water on and the machine off, check for drips at the inlet filter housing.
If That Didn't Work
The O-ring on the inlet water filter housing might be cracked or missing. This lets air suck in. Replace that O-ring.
Heads Up
Pulsing can also be a sign of a worn pump, but always rule out an air leak first. Running with an air leak stresses the pump seals.