Oh-likely the transformer described isn't issue with replacement Xantrex unit, my assumption was 10 year old factory installed unit.
Though any electronics can develop hum..I'm curious where exactly buzz is originating
I cant imagine the original converter/charger is active? Not really familiar with the Xantrex but my understanding is it does bypass/replace factory unit. The only reference to 'buzzing' looking thru Xantrex site was downstream GFCI not compatible with sine wave.
As its 2 years old, contacting Xantrex might be worth while IF still under 2year warranty
http://www.xantrex.com/power-products-support/warranty-repair-help.aspx
After the 2 years doesn't appear repair an option.
No opinion/experience with CW, would 'think' they might be able to trouble shoot your install, certainly could replace. Not sure about your RV 'mechanic' second time around?