What’s the quietest thing you’ve ever heard while working?

Today, after a near-miss with a misrouted ground wire, I stood in the dark for ten minutes just listening to the silence between the hums. Not the absence of sound—more like the space where the system breathes. It wasn’t safe, not really, but I stayed anyway. What’s the quietest thing you’ve ever noticed at work? Something that made you stop, not because it was loud, but because it wasn’t there.