It happens most often when I am alone. There seem to be two forms. When frustrated or angry about something, I'll curse quite uncontrollably with a seemingly random stew of profanity with varying syntax. Sometimes I sort of sing the profanity when I'm alone, not sure why. I'm too anxious around most other people to really let it fly, though.
Alone, especially in the shower, I find myself often having conversations with myself in other languages and pretending that different people are speaking in different languages and communicating. I'm sure it would probably be interesting if I was to record it and put it up on youtube...