I had a rattle in my One10 that turned out to be a little mettle bracket that the screws securing the driver connect to on the backside, which had fallen off and (being very light) was stuck to the driver magnet. I had to remove the cloth grill, then the driver, to locate the buzz (thinking it might be a bad driver which BF said it likely was - in fact they were so sure that they sent me a new driver which I did in fact install since 1) they were convinced and 2) it was there, delivered in 24 hrs from Brighton UK to the Chicago area). But I am 90% certain (like the google search doctor who is always near certain) that it was that bracket oscillating on the magnet when I played through it. I mean, it COULD have been a bad driver like Alex said (it was a week-old cabinet so maybe a unit faulty at manufacture by his supplier), but if so there was coincidentally also this loose bracket stuck on the magnet.In either case I'll never know for sure which since I took care of both things at once.
I must say Alex gave very good instructions for getting the grill off without harm, and while I nervously expected a difficult job, it was far less difficult than I thought. It went smoothly - though I was uber careful and followed the directions religiously. Bad driver or loose bracket, I never had that problem after that.
So- possible that your problem - and the sound of something falling inside - was one of those little clamp/brackets falling and becoming a free flyer from the driver; or, in your case, more likely a clamp that receives that screw inside, which might also be a small/light piece originally held by glue but meant to ultimately adhere by force of the tightened screw pulling it into/onto the wood of the cabinet.
Just my 2 cents, I am probably wrong, insert additional disclaimer here.
Unless it was touching the cone that little nut stuck to the magnet wouldn't cause a rattle.
