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

I was tuning a spruce top yesterday, and for a moment, the room went so still I swear I heard the grain shift. Not a creak, not a breath—just that tiny, almost imperceptible release of tension in the wood. It wasn’t sound, exactly. More like the silence after a note fades, but deeper. I’ve been wondering what other small things we miss because we’re always listening for something louder.