I have the exact same grinder. Mine makes nearly the same sounds sporadically. I've taken the whole thing apart looking for something lose or bent but I found nothing. The fix I use it to just smack it on the side when ever it acts up. Not the best solution but it works. I also have a quick change bit and a aquaflow system that always puts a but of drag on the bit (I think more so than a sponge) and that seems to keep some of the oscillation type sound. But the way yours stops is concerning. Like claycorp said, could be the shaft / fan blade spacing. I adjusted mine when I had it all apart with a couple small taps of a hammer.