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

I was changing a lock on a back door last week, and the client had just left. I stood there with my tools in hand, and for about three seconds—just three—the house was so still I could hear the air shift when I exhaled. Not even the fridge humming. Like the building was holding its breath. I don’t know what that was. But I’ve been thinking about it since. What’s the quietest thing you’ve ever really listened to? Not silence, but something that felt like it carried weight.