ac branch GFI circuits hum at receptacles with redundant gfi outlets, why?

[solen]

Yesterday, started using 8 AGM (24v 920amp) battery bank through Xantrex TR3624 inverter (with generator backup) to energize the 30amp ac service to house. All works very well, with the exception of a hum at the GFI receptacles in kitchen and bath. All regular ac outlet have no hum. Disable GFI at receptacle minimizes hum, Flip breaker at distribution panel no hum...why are these humming? The function when load is applied, but are noisy. Main ac breaker in dist. panel has a gfi breaker, so technically can remove the remote gfi at receptacles to standard, but i wannta know what is going on. Today will add PV arrays (126v max at 20 amp) and finish system test and start-up. Come Tuesday looking to pass state electrical inspection......any ideas, suggestions, or general interest?

[Mike90250]

Is this a mod sine inverter, or pure sine ? I suspect some oddity on the AC waveform, because GFI's have a tiny current sense transformer in them They won't like mod-sine.

[Sunking]

Make nailed it. GFCI have current transformers inside and what you are hearing is the modified square wave vibrating the coils. You will likely hear the same noise in any motors like your refrigerator compressor, fans, ect.. What is bad about it is it causes excessive heat and the vibrations wear the insulation out on the coil wires cause them to short out in time.

[solen]

Thanks so much, the inverter is mod-sine...bingo! will be removing the outlet GFIs after the state inspection. Spot-on!!