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

I was just sitting in the pre-op room yesterday, waiting for a patient who hadn’t arrived yet. The OR was empty except for me and the monitor blinking steady. No one spoke. The air was still. And then—just for a second—I heard the faintest click of the anesthetic vaporizer adjusting itself. Like the machine breathing. I don’t know why that moment stuck with me. It felt like listening to something ancient. What’s the quietest thing you’ve ever noticed, even when you weren’t trying?