warick_wrx

I have a Multiplus-II so far it is is pretty silent. It seems to make the loudest humming noise when connecting loads that typically generate harmonic's. Eg: The most noticeable is a hair dryer used at half its power setting (clipped sine wave). This is during normal and inverter operation.  Perhaps you could try isolate each of your loads in turn and see if the hum improves. This would rule out the possibility of load causing the hum.