Now for something that might be helpful.

Do any of your components have three-wire power cords? That means their chassis grounds are connected to the house's power ground. That can cause a problem ground loop, depending on whether there's current flowing in the ground.

How about the grounds of a cable, antenna, or satellite dish? Are they grounded? Are you sure? Are they well grounded?

If you get brave and ADD a ground from one half of the system to the other, what happens?

And be sure your baluns actually isolate grounds. (The origin of "balun" is "BALanced to UNbalanced," meaning basically a transformer, which incidentally would separate the ground on one side from the ground on the other side. People who are sloppy at naming things, in particular CCTV accessory companies, have infected most of us with the sloppiness of calling an adaptor a "balun" when the product is just hot and ground of a BNC to two screw terminals -- no balanced to unbalanced at all.)