I am going to divide the converter into two parts.. THE FAN and the rest of it.
THe fan runs on 12 volts and high heat. If the converter gets hot the fan comes on. Depending on who made the box it may well not care if it is "Plugged in" to shore/Genrator power or not, It is HOT and I'm Turning on the fan (or so says the Thermostat that powers the fan) This is the part you HEAR.
The rest of it runs all the time you are plugged in to shore or generator power It can not run without,