Loss of Flow and Pressure

My oldest little deck machine is having some issues, hopefully you guys can help. This is it:
(NorthStar Gas Cold Water Pressure Washer — 3.5 GPM, 4000 PSI, Model# 15782020 | Gas-Cold Water Pressure Washers| Northern Tool + Equipment")
Instead of 3.8@3500 I'm getting 2@600 and a LOUD knocking when I release the trigger. I checked the simple stuff.. water supply, oils, air filter, fresh gas with a touch of Sea Foam, and the carb's clean. I looked in the pump and didn't see any obvious signs of wear.
It sounds like the knock is coming from the engine not the pump, but I'm not sure. Any thoughts?

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