Diagnosis
A loud mechanical knock from the pump area is serious. It's often a connecting rod inside the pump that's come loose or is hitting the housing, or a severely worn crankshaft bearing where the pump connects to the engine.
How to Fix It
Stop using it immediately. This isn't a fix-it-in-five-minutes issue. You need to isolate the sound. First, remove the pump belt cover (if equipped) and see if the sound follows the pump speed. If it's definitively from the pump, the unit likely needs a pump replacement. These axial pump designs aren't really meant for user-level internal repairs on the crankcase. Continuing to run it will turn a pump problem into an engine problem if a rod breaks.
If That Didn't Work
Before condemning the pump, double-check that all mounting bolts are tight and that nothing external (like a loose hose or guard) is vibrating and hitting the pump housing. Also, ensure the pump and engine pulleys (if belt-driven) are aligned and tight. A loose pulley can make a horrific banging sound.
Heads Up
This kind of sound is different from the lighter "clicking" of a normal working pump. Don't ignore it. The forces inside a pressure washer pump are immense, and a catastrophic failure can send metal parts flying. If it's a loud knock, plan on a new pump.