Our old trailer had a squeal periodically from the (WFCO) converter. It was barely audible (e.g., the air conditioner or furnace would easily down it out), but it was definitely not the cooling fan running or the cooling fan going bad.
I noticed it primarily when we first plugged in after setting up camp. It didn't always happen and seemed to require a particular level of load on it, probably charging the battery after and a few lights, but not much else. It didn't seem to affect anything during the 70 nights we used the trailer over the past few years.
If that sounds about right, you can likely ignore it as well.