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

I was on night shift last week, and the line had been down for an hour. No machines running, no chatter—just the hum of the fluorescent lights and the occasional drip from a loose pipe. Then I heard it: the faintest click of a bolt settling in a cooling frame. Like something breathing under the silence. It stuck with me all morning. What’s the smallest sound that ever caught your attention at work?