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

It’s 1:37 a.m. and I’m editing a shot of an empty subway platform at dawn. The sound design is just the hum of a distant heater and the slow breath of the building. I keep thinking about how cities are never truly silent—there’s always something, even when you can’t name it. What’s the smallest, most fleeting sound that ever caught you off guard in a place that should be loud?