The Yamaha and Honda inverter generators have an "eco mode" that slows the engine when they're not heavily loaded. When being used to charge the batteries they literally purr. My Yamaha EF2000is is very quiet and doesn't work hard when my battery charger is putting out 45 amps in bulk charge mode.

If they were using it to power a pump or A-C or heater, then yes they can get to be two or three times as noisy. I agree people should be considerate of the hours and places they run them too. Never dawn/dusk or dark.