I do this when im feeling particularly impulsive (symptom of ADD/ADHD.) I just get the urge to shout out whatever nonsense crosses my mind, such as random words or sounds, like if I see a cat I might meow at it, but then I'll keep saying "meow!" to myself periodically. It got me in trouble a lot when I was younger. I don't have the desire to do it as much now that I am medicated.