You might try this. If you can, one night disconnect from park power, and see if it happens again. If no hum, could be an local electrical problem. Sometimes power companies will do load switching at night.
Also one night try turning off breaker for converter, and see what happens. That way you may be able to tell if it is a 120 volt problem or 12 volt problem with converter. If hum happens, then it is 120 volt system, if not them must be 12 volt system.
You would probably want to try this on a cool night when AC is not needed.