Ain't Poem

My memory is a bit hazy, but I believe I ran across something related in Dorothy Sayers, in one of her Lord Peter Wimsey novels, spelled imphm, or possibly imphim,   to indicate the casual nasal affirmative sound that results from saying "uh-huh" (meaning yes) with the mouth closed.  

Or it could be the kind of thoughtful descending hum that might indicate "Oh – yes, I think you're right" or "Okay, I get it now."  

In either case, it's an attempt to spell a sound that conveys meaning without actually being a word, if such a thing is possible.