As far as the voltage regulation on my UPS, it’s purely electronic so there’s no telling what kind of noise it might be putting back out towards the house.
I should be a little clearer - my local power utility doesn’t seem to do a good job with dealing with light-loaded periods. Meaning that instead of maintaining 120/240 volts, it might be 123/246 or something on that order. My UPS will, at some point, clamp the output voltage down a bit, and I think that might be when it is perhaps introducing some noise back into the house AC system.
I really haven’t studied it enough. My curiosity is only outranked by my laziness and the need to get other things done. :)
It wouldn’t take much to eliminate the UPS as a source/cause for the issue. Could also be the new electronic metering and fiber optic equipment that the power board installed on the house right at the same time. At any rate, it sometimes sounds like a very low-volume version of being in a power substation (where I’ve spent some time). Anyway, 3 years ago, the issue did not exist.