GFCI Hums

Electronic Components that hum are trying to emulate Bach's Fugue in G minor

They usually end up succeeding. After-which, strains of bag-piped Amazing Grace, can be heard through the smoke...

Well everyone confirmed what I was thinking. I liked your "They don't know the words?" SoundGuy......lol

The odd wave form is causing the issue.
A pure sign wave inverter would solve the problem.

They're running off an MSW inverter; you've (perhaps unwittingly) answered your own question.

MSW inverters by design do not output a clean sine wave, but a waveform with many high-frequency harmonics...some more so than others, depending on how well their output is filtered. This is causing something in the GFCI's circuitry to wiggle a bit and make noise.

Lenny K wrote:
The two GFCI's inside my Monty make a humming sound when running off of my Xantrax 1700 Watt MSW inverter.
Any ideas why this would be the case?

They don't know the words? :W

120vac fans I own which are designed to run on a sine wave signal hum annoyingly if I run them with an MSW inverter but those are motors obviously - not sure how or if this would apply to a GFCI but clearly yours don't like the MSW waveform. It's the reason I decided to bypass any potential MSW issue by instead installing a PSW inverter. :)