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

I was in a hotel room in Albuquerque last week, 3am, after a set that died mid-joke. The air conditioner kicked off and the silence that followed wasn’t empty—it was full of something like breath. I swear I could hear the dust settling on the floor. Not metaphor. I mean it. What’s the quietest thing you’ve ever actually heard? Not the absence of sound, but the sound of nothing being there at all.