Angelbear,
I'm not sure I can add much, but I would agree that it is probably a type of stim - specifically, a complex tic.
In my unprofessional opinion, any efforts to directly suppress these tics would be more harmful than good. Some method of redirecting them to a less obtrusive manner might help, all the while, some attempt should be made to identify what is causing the stress. I also agree that explaining that it bothers others is good.
My son used to yelp loudly while he would clap. He learned to blow on his hands as an alternative stim. It's quiet, it's physical, and it's "vocal!"
Hope this helps.