Diagnosis
That pulsing is almost always caused by air getting into the water supply side of the pump. The pump is trying to pressurize a mix of air and water.
How to Fix It
Start at the source. Make sure your garden hose is connected tightly to both the spigot and the washer's inlet. Run water through the hose with it disconnected from the washer to purge all air before connecting. Next, check the inlet water filter—it's a small screen inside the female garden hose connector on the washer. Unscrew it with pliers, clean any debris, and re-seat it with a little Teflon tape on the threads to ensure a good seal. Finally, with the machine off and trigger gun closed, turn on the water supply and squeeze the trigger for a few seconds to purge any remaining air from the pump and hose.
If That Didn't Work
If it still pulses, the O-ring or seal on the inlet side of the pump itself might be worn or damaged, allowing air to be sucked in even under pressure. This requires disassembling the pump head to inspect and replace the inlet seals.
Heads Up
Never run the pump without water flowing through it, even for a few seconds. That's called "dry firing" and it destroys the internal seals and pistons almost instantly, which will also cause pulsation.