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.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired