I had this issue years ago with my then-new 2003 330ci with only a couple hundred miles on it. The sound was coming from the right rear area. I took it in actually thinking it was a pebble and a quick fix. After a few minutes of waiting, I ventured to the service area where the car was to see my car on the lift, in the air, with the wheel off and 6 service ppl and my service mgr all huddled around the open wheel well and looking like they were scratching their heads. Soon thereafter my mgr came in to give me an update. He said that he doesn't know why it happened but one of the bolts holding the brake caliper in place had lost it's nut and the bolt was rattling around. He said he was going to get on the phone with BMW NA to discuss. That was the last I heard of that. Getting the bolts checked could be an option to try if you've hit a wall trying to figure it out. All the best!
Prodeco Phantom X3 eBike
2011.5 ///M3 Sedan || 2007 ///M Coupe || 2008 550i Sport || 2006 ///M3 ZCP || 2008 X3 3.0si || 2006 325i || 2003 Z4 3.0 Roadster || 2003 330ci || 2002 325i