Central AC compressors are 220V so both legs are polluted, nowhere to connect a truly separate circuit. Distribution transformer only has one secondary, so your neighbors' AC units will get you even if you shut off your own AC.
Remember that story last year about the Japanese audiophile who had his own utility pole?
A good passive filter, or regenerator, or iso-tranny should cure the AC noise.
Funny, I don't hear noise even when fridge and both ACs are running. They are all cheap 11yo whirlpool and carrier compressors.
I've noticed that people having noise or other AC problems are often in houses older than mine (11yo,) or in more urban areas. Could there have been changes in the electrical code over the years, like improved grounding or something, which have since stopped appliance noise from polluting other circuits?