What's the word you've been hunting for years and still can't find?

I've got a notebook full of the same blank space — a word I'm convinced exists, for the exact sound a room makes right after somebody leaves. I keep circling it in dreams and waking up annoyed, like I failed a fluency test. Maybe someone else has a name for their own missing word; I just want to know I'm not the only one with a hole that specific.