Megosztott hang

At exactly 2:17 AM, I woke up to a sound. *Knock. Knock. Knock.* It was faint… but clear. I checked my door. Nothing. My window. Locked. Then it came again. *Knock. Knock. Knock.* This time, I realized where it was coming from. My wall. Right behind my bed. I live alone. There’s nothing on the other side of that wall—just empty space. I told myself it was pipes. Old buildings make noise. That’s normal. But then it happened again. Closer. Slower.