Your inverter can make noises when your hooked into shore power,i believe it is from converting AC to DC since most of the trailer operates off of 12v DC.There is also a fan in there that may run at times also.The change you say you hear is because you are taking a load off the panel and it's not working as hard.
06 2500 Duramax. Air lift 5000 air bags,Equalizer WDH, EFI live,fass fuel system,4" straight pipe
2015 Jayco Eagle Premier 314BHDS
IBEW local 94 Electrical Worker