DIAGNOSIS That's the sound of cavitation—the pump is trying to run without enough water, or there's a severe restriction on the inlet side. It can also be a sign of a worn connecting rod inside the pump.
FIX 1) IMMEDIATELY turn the machine off. Running it like this will destroy the pump. 2) Check all inlet-side issues first: full water flow from the faucet, no kinks in the hose, and a clean inlet filter screen. 3) Make sure you are not using a hose that's too long or too narrow (less than 5/8" ID). It can't deliver enough GPM. 4) Start the machine with the trigger squeezed on the wand to purge any air, then release to build pressure.
IF THAT DIDN'T WORK If the sound persists with perfect water supply, internal pump components like the connecting rods or wobble plate are likely worn or broken. This is a sign the pump has reached end-of-life and needs replacement.
HEADS UP That knocking is literally the pistons slamming into the ends of their chambers. A few seconds is bad, prolonged use will grenade the pump. Always address water supply first.