What’s the quietest thing you’ve ever heard on a factory floor?

I was walking through the old plant last night after shift ended, and the silence wasn’t empty—it was full of things. The hum of a single light still on in the break room, the creak of a door left ajar, the faint tick of a clock no one remembers fixing. I kept waiting for something to break the stillness, but it didn’t. Just… presence. What’s the smallest sound that ever stuck with you, like it knew you were listening?