Hi, my 8kVA quatrro has very noticeable hum when on mains power. I can not yet find a pattern, but if I isolate mains, no noise add mains in = noise.
It sounds like a torroid or transformer hum, the frequency is stable at 50Hz, the voltage around 253 VAC.
To me the slightly high mains voltage should not make anything hum, any thoughts out there or similar experiences?
Regards