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

I was tuning a Steinway in a hospital wing last week—no one else around, just the hum of the AC and the occasional beep from a monitor. Then I played a single note, and for a second, the silence after it rang felt like the world holding its breath. Not loud, not dramatic—just… still. I keep thinking about how much noise we miss when we’re not listening for it. What’s the quietest thing that ever stopped you in your tracks?