You're probably hearing the "converter" fan running if plugged into shore power. Your lighting, slide motors, control boards for the fridge/water heater & the furnace fan all run on 12 volts so once plugged in the "converter" supplies this power while also charging your batteries & is equipped with cooling fan that will run when required. A "converter" changes 120 volts to 12 volts & is a battery charger.
An "inverter" uses your onboard batteries as the power source to change the 12 volt to 120 volt to a dedicated plug or two, typically to a residential fridge if equipped & possibly the tv.
Danny & Linda
Sold '13 Thor Redwood 36 FB
Traded '13 GMC Denali DRW D/A
Currently RV & truckless