Electricity only makes a sound at a 50 Hz or 60 Hz pitch.. should be very deep but constant hum. Radio frequencies go way above what people can hear, so it's none of that stuff.
Some speakers without EM shielding buzz a bit when there's a EM wave nearby, like from a cellphone, or another speaker. If you get two TVs doing the same thing, it's probably interference from some EM emitting device. Cellphones or magnets or something like that. If you've got one of those magnetic wrist bands, that could be the culprit, lol.