I remember in the late 90s when I got my first job as a web programmer at my University. I walked in to this basement-cave with several of out of central casting uber-nerds sitting deep in thought in front of their monitors. I remember getting myself set up and being really surprised at the cacophony of mutterings, celebrations, and chuckles that these guys emitted for no-one in particular.
I worked there for several years, but never really joined in with this behavior, maybe I didn't see myself as being uber enough to warrant this (what I thought of at the time as) autistic-like behavior.
Fast forward almost 20 years, and I do this all of the time (though admittedly not in the company of others). I'm not sure when I got into the habit of it, I think it's been the thousands of hours of development I've done by myself, where it seems to serve as a way to keep myself motivated, on-task, and in flow.
Is this something you do, and is this a unique behavior trait to developers? I can't think of any other profession where it would be socially acceptable..