I feel like every story about a creepy apartment I read winds up here. I love how the narrator of the story slowly reveals more about his own psychological state.

It’s not worth knowing what’s inside the wall. The thing’s brick and I’m not made of money, especially not after buying a home. The knocking you hear, coming from behind it—just ignore it. It’s the big city; apartments always knock. It’s the air conditioner in the place next door, maybe. The pipes, probably settling. It’s nothing to concern yourself with as a first-time home (apartment) buyer—

It’s just a vaguely human shape painted on the wall of one of the bedrooms, that knocks at specific intervals.