Sounds like it could be the inlet check valve. Does it hum when using onboard water only? On my 35B, I currently have a humming when using the hot water - pretty much narrowed it down to the mixing valve on the back of the water heater. Hopefully, it won't fail. Only problem is I can't get to the back of the water heater without taking it out. If necessary I may cut a hole in the bed platform for access. Will then have to rig up a way to secure the cutout when finished.
2015 Fleetwood Excursion 35B; 2012 Suzuki Grand Vitara toad