[wingnut60]

Should be the cooling fan that comes on when the CONverter is operating. I don't think it is a problem.
It is designed to keep the battery(s) charged--whenever you pull power out of the batteries (lights, water pump, etc) the converter will turn on to replace the power (recharge) the battery(s). It will not operate if not plugged into outside power.
If you ever have a failure of the converter, consider replacing it with a Progressive Dynamics series 9200--these are the best on the market and automatically maintain battery charge without danger of overcharging.
Might want to check the battery conditions also if they are not new.
Joe