I have (hopefully, *had* ... read on...) this "cow bell" noise as well, on my 6 month old E93 M3.
Here's what I heard : http://sdrv.ms/QimgDd
I had previously taken it into BMW Seattle twice ... and both times they took apart the wheel and brake assembly, not finding anything untoward, no stones, etc and put it back together again ... only for the noise to come back.
I had intermittently experienced it, perhaps a couple of times a month, until this week when I heard it every day, so I booked another service appointment.
Today, my SA (Nathan) at BMW Seattle immediately called in their shop foreman, due to the recurring nature of the issue. Of course, we couldn't repro today on the car - but the shop foreman watched the videos I took this week, and he *IMMEDIATELY* said "I know what it is. Ive heard this before. It's loose rivets in the brake rotor."
They ended up replacing the rotors and pads on both rear wheels, all under warranty (remember, I collected the car 31/Dec/2012, so it's less an 7 months old with only ~5500 miles on the clock).
I'll continue to monitor for it, but hopefully this will be the end of it!! Fingers crossed!!