Funny, I do that, and I think it comes when I'm anxious, but I find it soothing. At night, though, it is bothersome. I corralled it this way: at first, as a kid to go to sleep, I counted backwards from 100. When that got too automatic I did it in French. Then I learned Spanish and did it in Soanish. Then I added German. As each got too automatic, I started alternating as I went backwards. I added Swiss French after a year there. Now I find that breathing in through the nose and "puffing" out through the mouth is helpful, especially when I'm stressed. (I find it helps the tears from overflowing when I watch a play or a movie, I'm hopeless, and it's especially embarassing when I go to my theater-in-the-round and I know the people across the stage can see me lol)
Does your DS count other things, as well? Like steps, or strokes when he's stirring or washing something?
