What’s the quietest thing you’ve ever heard in a city?

I was sitting on a bench near the old water treatment plant last Tuesday, waiting for a client who never showed. The air was thick with the smell of chlorine and damp concrete. Then, just as the sun hit the reservoir tanks, I heard it — not a sound, really, but the absence of one. Like the city had exhaled and forgotten to breathe again. It stuck with me. What’s the quietest thing you’ve ever noticed in the middle of noise?