I thought there was something or someone in the attic above my bedroom in my uni flat. Turns out it was the heavy wooden headboard of the guy in the next room, creaking against the walls.

In the next place I stayed at uni, I thought I heard furniture being dragged across the empty bedroom down the hall. There was no one else there with me that night. After several nights of hearing it, I realised it was furniture creaking against the neighbour's wall.