After a few nights of wondering if I was going insane because I was hearing people walking around above me (I live on the top floor) and in the walls, I finally figured out - it's an animal! There must be some kind of animal that lives in my wall. It's most likely a rat or a squirrel, and I had a bunch of dreams that they ate a hole in the wall (which is apparently fairly likely) and a giant rat burst out... so I guess I'll report it to my apartment complex and see if they can do anything about it before they come tumbling out of my wall.
