I'm autistic + ADHD, so I have all sorts of repetitive-motion stims including but not limited to finger-tapping that are *probably* not connected to the stutter. But, y'know, you never know - and apparently there's some evidence showing stuttering may be more common in people with ADHD. That said, your Google search is actually showing me some other research which I too find really interesting and possibly indicative of the underlying issue - the fact that there's more and more evidence showing that adults who stutter often have an underlying deficit in producing a rhythm in other ways, and IIRC some research showing that this rhythm deficit exists for kids who will go on to stutter as adults but not those who will recover. Anecdotally, this tracks, I've always got the impression that I have very poor rhythm skills... and my mother is fully convinced of it, lol. Apparently I had a phase as a small child prior to the actual stutter developing where I would talk like a waterfall while running in circles but stop the instant someone picked me up, as though I needed an external pacemaker to maintain my speech; looking at videos of me as a kid, I actually started *stuttering* fairly late (five, I think?) but there is something clearly off about my speech rhythm from pretty much the start, with uneven syllable lengths and weird pauses. So "underlying difficulty in developing and maintaining bodily rhythm, whether that's an issue with generating the rhythm at all or translating it to movement" is my own pet theory, whatever the cause for that is (and, like, the genetic causation research on stuttering is *wild.*)