Yeah, this one's common. The pump is sucking air because the water supply is restricted. It causes a pulsing rhythm.
First, turn the water source on fully. I mean, crank it open. Then, walk the entire length of your garden hose and straighten out every single kink. Also check for leaks where the hose connects to the faucet and the pressure washer.
If that didn't work: Your hose might just be too skinny. This unit needs a good water volume. Swap it for a 5/8” or 1” diameter hose. The skinny ones you use for watering plants can't keep up.
Heads up: That pulsating is hard on the pump. If you keep running it like that, you're headed for a service call. Always start with the water full blast and a proper hose.