I've been noticing it also.
I live on a hill, no machinery,
I've listened when everything is off, so it's not coming from inside my house.

It sound like an electric motor running.

I live a long way from cell towers, a long way from town.

At first I thought it was coming from the basement,
there is a lot of ledge in this area,
but I don't hear it down there.

I only hear it in the north end of the house.