Your pump has residual friction from sitting unused. The components are stuck.
- Disconnect the water supply hose.
- Plug the unit in and set the power switch to ON(I) for 2 to 3 seconds.
- Turn it off.
- Repeat this a couple of times until the motor starts normally.
If that didn't work: You've got voltage loss. Unplug any extension cord and plug the pressure washer directly into a 120V outlet. Make sure nothing else is running on the same circuit.
Heads up: If it still hums after trying both fixes, the unit hasn't been used for long periods. You'll need to call customer service at 1-888-909-6757.