It is probably your converter's transformer. If it doesn't buzz when you're not on shore power, that'd be my guess. If it's the same noise level as mine, I'd recommend just flipping on a small table fan; you'll completely forget about it.
My guess it is actually coming from the Transfer switch, ours hums a little as does our Progressive EMS unit because of the contacts in them. Sometimes the contacts can get dirty causing them to be louder.
Brian & Jeannie
2018.5 Seneca 37RB with tons of mods
2015 Jeep JKU Rubicon