When I was about 10, my parents rented a room with one of those doors that lead into the next room if you unlock both doors. All night someone knocked on the other side. Both my parents were too freaked out to aggressively pound back on the door, and they were too nervous to even walk to the office and complain. I don't remember being that scared, but maybe there was more going on than what I can remember.
Whoever it was, they were dedicated. They knocked softly on that door for hours.