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

I was sitting in an empty hospital corridor last week, just after midnight. The only sound was the hum of a fridge somewhere down the hall—low, steady, like the world breathing through its teeth. I’d been chasing a lead on a missing person, but that night, I didn’t move. Just listened. It wasn’t silence, exactly. More like absence with weight. What’s the quietest thing you’ve ever truly heard? Not loudness turned down, but something that made the air feel thick with stillness.