It's never at the same time, it's not any of the televisions in the house, and if I move from the spot where I'm sitting the sound stops. It's not loud, just at the edge of my hearing. And I haven't heard it anywhere else.
Let me tell you, that's kind of creepy when you're the last person awake in the house and it's 1 in the morning.
(Also, it's driving me nuts because I can't figure out what might be causing it.)