Originally Posted by AirEPotter
Converter is behind the panelboard. I am thinking its a cooling fan. It only makes the noise when I am connected to shore power.
There is a small fan that is part of the converter. The speed is supposed to vary according to the temperature of the converter. Normally it should be barely audible but as it warms up in there, the speed picks up and a noticeable fan noise will be heard. The bearing in the fan is not designed for longevity but from the age of the trailer in your profile, it sounds like it may be prematurely failing if it sounds like a bearing noise. As previously mentioned, try vacuuming out the converter to remove a possible layer of dust that may be retaining heat and making the fan speed up. Canned or compressed air may also be helpful to blow it out.