Humming noise when on city water and tap in MH is turned on

I've solved that humming just by turning down the city water valve, just a little bit will do it. but that may be just one of many solutions.

Bad check valve

Yep. Check valve. We replaced ours and guess what? The new one was bad too. Had to replace it again! Now no humming.

City water inlet connection has an internal check valve....when they are failing they will start humming. Replace.

And could be water pressure regulator......crud built up on valve/seat area.
If yours is rebuildable just disassemble and clean out internals. Otherwise...replace.

Also can get a humming sound from water heater outlet (check valve) when it is failing. Replace

ON EDIT:
Noticed you have 'Yuma' listed as location.
Very hard water....you have crud built up

If you are going to be in Yuma area much (or Southwest US) might want to invest in a portable water softener......several brands (On-The-Go, FloPur, Water Stick)
They will stop that build up...check valves, faucets, toilet water valve, Water Heater etc.

I would guess it's your water pump left on.

Harmonic sound from restricting the flow of water, you may want to either increase the pressure at the regulator or decrease it just a bit (if you have an adjustable regulator).

Could be your water pump. Should be turned off when hooked up to park water.

It could be humming because it doesn't know the words. :D

More than likely the city water input valve (on the coach) is shot. Best to replace it. Get one at any RV store.