It is the cooling fan for the converter. It is thermostatically controlled. The more load on the converter the more the fan will run. When you disconnect from shore power the converter is no longer being used.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.