IMHO, at this point I think you might be chasing your tail. Most LF inverter builds appear to have have some slight noise at idle at random cycling occasions, some other / older designs can be quite noisy in the Toroid and chokes, you will definitely get noise from the Toroids when unbalanced loads pull more power from one half of the AC cycle / cause slight DC injection into the AC line with associated harmonic distortion.

Ok, good. Because I've been dealing with self inflicted balance issues I'm probably being a bit paranoid about there still being a fault lurking somewhere  

If I could ask another question then:

How much difference in frequency is there between 50hz and 50++? The reason being with both of the nanos I've tried the frequency is about 49.87Hz and I'd like it to be a bit higher if possible, but I tried 50++ and didn't notice much change.  Do I just need to try different nanos?

I don't know if it will matter yet TBH as the house is still running on the powerjack (50.002Hz), and it may be there's nothing using it as a reference, but at least one of my old oscilloscopes uses a ferroresonant transformer and needs a supply reasonably close to 50hz.

When you change the frequency in Setup, the change will not be acted on until the Nano controller has been fully powered down and restarted. The exact frequency depends on the Xtal on each Nano board, it varies "slightly" with each Nano  
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Edited 2025-08-07 12:10 by KeepIS