I notice this happening a lot, and I always wonder what the brain is doing, how the brain benefits from all of this. "This" being, as I got my dinner out of the fridge tonight, I became acutely aware of a little voice inside my head, creating Spanish phrases while I scarcely paid attention. Today it was:
... a mi, a mi, a mi, a mi me encantaría decirte una cosa.
... I, I, I, I would love to tell you something.
At least it made sense. Sometimes it makes syntactic sense (word order, right parts of speech, etc.) and not actual semantic sense.
The frequency with which this happens correlates to how much Spanish I've been using/hearing. The more I practice, the more vocal my brain becomes when I'm silent.