This is all so interesting! My DS10 has always done something similar. He is an incredibly creative kid, and tends to leap around (he jerks his hands up over his head when he does this, and often points his head downward sort of... its the same movement most times - but its not rhythmic at all... does that rule things out for this?) and make odd mouth noises (I call him explode-a-boy when he's doing it, b/c it sounds like he's making explosion sounds).
It comes and goes, and I haven't seen it for a while but right now its back. Its great a) to hear we're not alone in this and b) to have something new to explore to learn about it. We thought it was more OT related, and sensory seeking behavior -- now reading this, SMD fits MUCH more than with sensory issues (he is a sensory kid - loves strong flavors, amusement park rides, etc., but things that should help a kid with a proprioceptive sensory disorder [seeking the pounding sensations that would follow his jumping around] don't really seem to make much difference for him).
For DS it started young. We have a video of him at 2 running back and forth in and out of the room where DH and I were sitting telling us a story he was making up. ;-)
We've always worried a bit about it for social reasons - and more so now that he's getting older! - but he's also slipped twice while doing it and once nearly cracked his head on a metal manhole cover. It was sheer luck that he didn't...
