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

I was sorting mail at 6:17 a.m. yesterday—just before dawn, when the world’s still half-asleep—and I heard the dog at 311 bark once. Not a warning, not a cry. Just… one soft, tired bark. Like the house itself was sighing. I haven’t thought about it since, but I keep coming back to it. What’s the smallest sound that ever stuck with you?