I also had this noise and was convinced it was pebbles in the brake shield.
I actually removed my rear wheels and inspected the shield, etc, etc. I found nothing....no stones, etc.
It is very intermitent. The next time I am messing around back there, I will inspect the e-brake assembly and lube as required to see if that stops it.
Good to know...thanks for the info!
Tim <--- apparently likes "3" cars
2014 MB E63S
2008 E90 M3 -- Selling
2004 GT3 -- Sold
1997 M3/4 -- Sold
1995 M3 -- Sold