Well, it could be a bit of the pad sort of shorn off and lodged in there somewhere. Then again, a sticky wheel bearing or rubbing dust cover or a little chunklet of road debris might also make that sound. I presume you've already had that first 300 mile service done, but how far's the dealership? Were it mine, still under warranty and all, I might bring it on back for them to give it a looksee. You've already got a great little film perfectly illustrating the symptom, which is already 90% of the way towards its diagnosis. I mean, you won't just be standing there in the service area telling them it makes a squeaky noise and then suddenly not be able to make it do it, which would be par for the course, right? You won't have to be that guy standing there in his leathers going, "Eek!...Eek!...Eek!"