This is quite interesting. I don't repeat words or make them up, but I talk a lot to myself when I'm doing household chores. In different dialects and voices, or sometimes in rhymes. My brain finds rhymes very entertaining for some reason.
If I like a spontaneous rhyme, I might say it again when I repeat the activity that inspired it. For example, after drowning my first cup of coffee in the morning, I might say something like "I drain this cup of beverage, my brain is up to average." It's totally silly, I know But since I'm alone 24 hours a day, I need to do something to keep the old brain in working order and myself from going crazy. At least that's how I used to rationalize it until now. I never thought of this as stimming, but it makes a lot of sense now that I think about it.