J.H.
My 9 yr. old step-son does the same thing. He started this at a young age with repeating words. I just constantly remind him the stop, he doesn't reallize he does it either. I have to make him see what he is doing to stop it. Usually changing the pace, giving him an activity to do will stop it for a while.
With him I truly think it is boredom. At his moms he constantly plays video games and has since he could hold a controller. We don't have any games here at all. It seems he just doesn't know what to do with himself without playing games.
Good luck and let us know what happens.