migtig
aka Mrs. Giant
I've had this happen, turned out it was the door bell box thingy going bad... you know the box that hangs on the wall. Ours is on the ceiling and it does it from time to time.
Same thing here. It was intermittent and for the longest time it seemed like I was the only one in the house (along with the dogs) at those times to hear it. It was driving me bonkers. I had no idea a doorbell did that. The giant finally heard it and replaced the doorbell box. Once he replaced it, I was surprised by how quiet the house became and realized that thing had been emitting a low level noise the entire time it was going bad.