They will do that if there isn't enough pressure. First, leave the hose slightly loose at the washer and let all the air purge out of the hose for a few minutes. Then try to run it.

With any power washer you want to make sure you have water not air running through it as suggested. It's always good idea to first run all the air out of the water hose before you attach it. then attach it and turn the water on. Now pull the trigger and let the water pressure push all the air out of the unit. Now you are ready to plug it in and turn it on. I also don't recommend let it run very long without the trigger pulled. I know some have pressure shut off valve that turns the elect unit off, but it they are still humming they are still pumping. I never let my gas unit run very long without the trigger pulled(less than a minute) without shutting it down IMO not good on the pumps.

My pressure washer did not turn on. Replaced the plug with another similar GFCI plug. Works perfectly!