I had it happen to me years ago.

It was a high pitched electronic whine, everywhere in the house, much louder near the ceiling, but nothing in the attic, nothing outside.

Of course it was 3:00 AM. I probably spent a half hour going from room to room with a stepladder trying to isolate it, going in and outside just to see.

Then I was outside scratching my head, and switched hands. There it was. My battery operated wrist watch was making it. When I climbed a ladder it echoed off the hard plaster ceiling and got louder. Outside it just was absorbed.