I knew someone this happened to. Several months after moving into their new house they started working on a neglected secondary bathroom on the 2nd floor. They didn’t go back there too frequently because it wasn’t a priority, but it was a really cool space connected to their bedroom via a small dressing room/closet.

One day she noticed a low, humming drone and checked the wall of that closet. She claimed that the whole wall was warm to the touch and vibrating. That was not an easy or cheap job, but we got to make jokes about bees for a couple of months.