You know what when you stick your tongue partway out and go “pppbbtthhhhh” you are “Blowing a raspberry”—right?
SO-
If you place your index finger lightly across your lips, in a horizontal position, and rapidly jiggle it straight up and down while making a noise, you go,“bupitabupitabupitabupita” WHAT IS DOING THAT CALLED???
(I did explain to her that “raspberry” was from the Cockney rhyming slang for “fart.”)
I don’t think it’s a trill, because (I think) in a trill, the vibration is caused by air passing over/between the elements – e.g., in the case of the linguolabial trill, the lips and the tongue.
In bibbling (that seems to be the common term for it), the vibration is caused by the finger.
I spent a little time trying to track down “Bibble,” as a mouth noise. Alas, nothing rose to the bait. It’s the name of a software company, and that got in the way. As a slang term, with widely various meanings, it seems to have sprouted from Snoop Dogg’s enigmatic shizzle-talk.
My old 12 pound Webster says *bibble-babble * is idle chat. It says a bibber is a man known to be a drinker, leading me to remember a moment in the movie Amadeus.
*Mozart was at his pool table. He was writing down music, bouncing around a pool ball, and drinking wine. A fellow (Salieri?) comes in to ask if he has finished a piece of music he has promised. Mozart says it’s finished, pointing to his head. “The rest is just scribbling. Scribbling and bibbling, bibbling and scribbling.” * I understood his “bibbling” to mean drinking.