Diagnosis
You're likely dealing with a pressure washer that's air-locked or has a clogged inlet filter. The hum means the motor's getting power, but water isn't flowing through the pump properly to build pressure.
How to Fix It
First, make sure your garden hose is on full blast and there are no kinks. Then, with the washer off and the trigger gun closed, disconnect the high-pressure hose from the pump outlet. Turn the water on and let it run for 30 seconds to purge any air from the pump. You should see a solid stream of water come out. Reconnect the hose, make sure your spray tip is unlocked, and try it again.
If That Didn't Work
Check the inlet screen on the end of the washer where your garden hose connects. Unscrew it and you'll find a small filter. Rinse it clean under the tap—debris there can starve the pump of water.
Heads Up
Never let the pump run dry for more than a few seconds. That humming is the sound of it trying to pump air, and that'll kill the internal seals fast. Always get the water flowing first.