What’s the quietest thing you’ve ever heard in an empty room?

I was just standing in the studio this morning, after the last rehearsal, and the silence wasn’t empty — it was full of something. Like the air still held the echo of a tendu, or the ghost of a breath between phrases. I’ve never heard anything like it. Not even the hum of the fridge. Just… presence. What’s the quietest thing you’ve ever noticed when everything else has gone? Not silence, exactly. Something older.