When I started school I was a continuous flapper, using both hands which were held fairly high. This was in the dinosaur age, before psychologists and diagnoses, so of course my flapping was deemed inappropriate. It took teachers about 5 years of serious effort to rid me of my flapping, but then I started incessantly talking in class.
I think this was my replacement stim. I would get sent out of the classroom and got hundreds of detentions, but when back in class I would continue with my chatter. For years I wondered why I did that, knowing that I wasn't angry or defiant. Then one day I read a comment by an AS who said that he stimmed by talking, and the light went on. Since then I have read a comment by a different AS whose school experience mirrored mine, including getting removed from classes for his continual talking.
I should add that my comments were each different, i.e., not repetitive, but I am sure that they were a way of stimulating the senses (or a way to prevent me from completely zoning out in a daydream, which I was prone to do?).