I do this all the time. Literally every day just about. When I was younger it was only when something was REALLY bothering me. Or if I really wanted to tell someone off, but I'm too much of a passive aggressive person to do it to their face.

But recently, whenever I find myself alone I'll do it involuntarily. More recently, I've even started doing this when I'm not alone. I'm one of THOSE people that you see walking down the street talking to themselves. It's actually a little shattering when I realize I've been doing it and not realizing it.