For what it's worth, I've had this problem in my 2011 E92 DCT.
It's not a rock or debris related issue.
It is the parking brake adjustment.
You can make the tin-can clanking sound stop by rolling slowly and pulling up on the parking brake several times at low speed. It will then likely go away, but return again in the future.
This was verified by my local BMW shop.
The fix is to take your car into the shop and have them adjust the parking brake.
Incidentally, the sound is very distinctive, and does sound like cans being dragged behind the car. Kind of a hollow, thin-metal (like a can) clanging sound. Very annoying, and usually only happens at parking lot speeds. (and when others around to witness
Good luck.