Wow I did not expect so many people trying to help me so quickly.
Seems like this is the right forum to post such a question.
I'm not going to tell my therapist about it though, I feel it is not a problem to me because I can fully consciously control when I do it. Also when I really want to do it I can tap my finger in a descreet manner (and I sometimes do when I am bored on a train for example).

I'll report it in the Autism subofrum as advised. It makes more sense it being related to that rather than OCD, although I was never diagnosed with any form of autism.

I don't think it's a way to dissociate as I was never abused as a child, but I get the logic behind that idea as well.
It seems this is quite a rare thing, but I suspect it is almost certainly related to a more common mental problem because I am sure my cousin has/had the same thing as me.