Diagnosis
This is almost always the pump being air-locked. The pump can't build pressure if there's air trapped inside instead of water, so the motor just strains against it.
How to Fix It
You need to purge the air. First, make sure your garden hose is on full blast and connected tight. Then, with the pressure washer off and the spray wand trigger locked in the open position, turn on the water supply and let it run through the machine for a solid 30 seconds. You'll see a steady stream of water come out of the wand. That's pushing the air out. Turn off the water, connect the high-pressure hose, turn the machine on, and try the trigger again. It should catch and start pumping normally.
If That Didn't Work
Check that the inlet water filter screen isn't clogged with debris. It's the little screen inside the garden hose connector. Unscrew it, rinse it clean, and screw it back in. A clog there can starve the pump of water, causing a similar symptom.
Heads Up
Never let the pump hum like that for more than a few seconds. It can overheat the motor. If it doesn't catch right away, shut it off and go back to the purging steps.