I believe the fan runs when it is charging at a high level to keep it cool. Sounds like your battery is fairly low, see if the fan stops after a while, how long depends on the state of charge.
If it keeps running, then it may have gone bad. Had one that came on whenever it was plugged in and stayed on on my old trailer, replaced the converter to fix. That was an Elixer which I replaced with a WFCO.