Speaking of “hum” and harmonics - true, but weird story - I bought a new UPS for my desktop computer a couple of years ago. The night after I first plugged it in, I started noticing a low-level low frequency (60 Hz) hum in the house. Not all the time, but the noise coincided with times when power demand was lighter. At times of higher demand, the noise doesn’t occur.
I have never been able to pin down a physical source of the hum; only that I first noticed it when I got the UPS. I think that there’s some connection between the noise and the UPS though. On the other hand, I have a 161,000 volt transmission line running across my property, about 250 feet away from the house. And a nuclear plant about 6 miles to my northeast. Now maybe the line is causing some standing waves, or maybe the voltage controlling circuits in the UPS are interacting with the household supply in some odd way.
It’s been an interesting phenomenon, but not bothersome enough to force me to do something about it. Just a curious oddity.