My guess is the noise you're hearing isn't the inverter (or converter, more likely), but an automatic transfer switch that should be wired between the generator and shore power connections. The transfer switch prevents the generator from trying to backfeed the shore power. It's probably bad, or at least really dirty and sticking. In my B190, the transfer switch was wired prior to the breaker box, so the breakers wouldn't have an effect on it like yours seem to, so I'm not entirely sure this is the correct answer, but it's the first thing that made sense to me.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab