Problem with AC and how to solve it

It is probably not a voltage problem at all. You are carrying noise on your lines from something. It could be as simple as a light dimmer or something similiar.
Some sources of lighting will cause it. Probably need to turn things off in your home until you find the culprit.
As long as the AC is clean, slight voltage variations don't mean much (other than affecting the maximum output of the amp.) Most power companies operate in a + or - 10% realm. It is not unusual for voltage to drop down to say (maybe even lower) as low as 108v (sag) in the summer and then peak up to as high as 127v or more at other times. If your voltage is outside of these approximate values, you will need to contact your power utility and see what is going on. It will effect your refrigeration and air conditioning systems adversely also. Actually anything with motors. When voltage sags, amps increase and when voltage increases, amps decrease for a given load. Fortunately, most motors have a service factor built in that helps motors survive unusual situations. Over voltage can be as bad as under voltage.
A severe under or over voltage would be required to cause noise in a transformer, if then. I feel confident your amps are suffering from some sort of RFI or other noise on the line. It could be coming from about anywhere.

Or it could be that there's DC present in the house power wiring at certain times of the day. DC will cause a transformer to buzz. This is a power quality issue; there are power conditioners out there that can address the problem such as PS Audio, Furman, and others.

Audiophile APS 1050 regenerator- will output a perfect 120 v/60Hz sinewave when your wall voltage is anywhere from 80-144 volts. It should solve all of your problems if you feel they are related to flucuating line voltage. The 1050 is also a UPS and has short term battery power usage, with additonal battery packs available. It's a very good unit- I used to have one.

http://www.audiophileaps.com/