Hi PsychSearcher. I can say that I do a similar movement too, though I don't always imagine something during it. I too do it since I was a child, though now I reduced it a bit. I've always considerated it a tic.
I don't have a therapist and I'm not diagnosed with anything yet, but I really think to have OCD too. So maybe the two things can be related. But I'm not sure.
I think that you could talk about it to your therapist. He wouldn't judge you, and he could tell you what to do with it.
Feel free to message me if you want to talk. We seem to have something in common.
Welcome to PC I'm happy that you're here.