The small ones are brief periods of receptive and expressive aphasia. I imagine the only other people that have felt similarly are brain damaged stroke victims.
The more intense ones feel like being in a crowded bar, but my auditory filter turns off and I'm suddenly having a hundred conversations at the same time (but with my inner monologue, there's no auditory hallucination). It's like an overwhelming vortex or swirl of words and understanding. It's also pretty terrifying because it means I might slip off the edge into unconsciousness. Fortunately, they're rare. Also unfortunately, they're random.