When I'm alone, yes. I also do many other "strange" vocal things, such as humming or whistling for really loudly for long periods of time because I HAVE to hear music out loud sometimes. I suppose it has to do with hypersensitivity and my extremely high musical aptitude. There's also the ease with which I can, like you, imagine a participant in the discussion.

It's, uh, not normal. At least in terms of people with lower intelligence having little or no desire to be creative.