If it were a ground loop the buzz would be coming from the speakers, which it isn't at all. It's mechanical buzz from the transformer. At this point I'm pretty much convinced that it's the transformer's response to fluctuating AC voltage.

Walks like a duck, smells like a duck looks like a duck, it's the TV Dude. Are you using the TVs speakers at all? If so, turn them off in the TV menu. What type of lighting does the TV use. It could be a bad inverter in the TV backfeeding into your AC circuit.

TV speakers are off. TV is a plasma. It's the TV in the sense that the TV is causing the voltage on the AC line to vary substantially. All the evidence points to that as the cause now. Makes complete sense. Probably would not have come up in the winter, when the grid and the house circuits are not so heavily loaded. Right now, I sometimes see the voltage to the A/V system drop as low as 111 volts, with the current up to as much as 6.7 amps, depending on what's on the screen. If it were noise on the line, I'd expect it to affect other components as well, and perhaps to manifest in the signal feeds, which doesn't happen.

Having said all this, can you call a troubleshooter..ie expert electrician who would be able to fix something in your lines and give the tv a dedicated line. I don't know if this will work. Because if you hear this noise in the quiet parts of the movie then there is a problem that will always bug you. Can you somehow TEST a different tv on your system. I know it is a huge job BUT maybe the tv has got to go. First get expert electrical advice.

I have an APC H-15 which maintains system voltage right at 124 vac where I have it set. You can get them online for $2-300. I actually got both of mine for less than $100 each a few years ago. They also provide great lightning protection. You can also get them with battery backup.

Fortunately there's a socket connected to another circuit nearby, so I'll just feed the amps from there. That effectively fixes the problem.

Yeah, I may go to something like that eventually. Thanks for the lead.

Maybe you can extend that line close to your system so you won't have to run an extension. Nice to keep the thing neat and tidy. The APC conditioner Xenonman suggested is worth the try.

Try checking the circuit at the breaker for any indications of overheating or loose connections. That will be easy to do. A loose connection on the circuit or its ground could be causing the problem. Turn the breaker off and check all of the receptacles fed by it for tight connections and good grounds. Pop the breaker out and replace it with the same style and amperage rating. The breakers are only like $6 at HD or Lowes and take 10 minutes to swap out.

Have you checked the internals of the 7125? It's possible the transformer is not bolted securely and that this is causing the buzzing. Or maybe something else is loose and vibrating under load?

It you open up the unit be careful of the caps. They may hold charge long after the unit has been unplugged.