Your converter/inverter is probably in an outside compartment. Mine is in the compartment under the head of our bed. When I plug into shore power, I can hear a slight hum when the charger kicks in, but I don't seem to be able to hear it inside (but then again, hearing isn't what it used to be
) You may be hearing the charger cycling on and off.
Under the bed (on mine) is the transfer switch, which switches between shore power, generator power and converter/inverter power. I've heard a click when the switch engages.
Were you connected to shore power or boondocking?
I've heard a hum coming from the plug in the bathroom near the sink when boondocking, but only when something is plugged in, like the hair dryer.