Check this thread and you will find more information about it.
Trust me, it is VERY annoying and seems to be a fairly common problem. My dealer has done everything from cleaning out the rear drums of the parking brake, cleaning the brake pads, to changing the pads, sensors, and rotors. Each time the problem goes away for a few months and then starts back again.
I still wonder if there is a permanent fix to the problem. I have seen complaints from owners of 330's and 335's, so it doesn't seem to be limited to only one of the braking systems.
2008 BMW 535i Space Gray, Black Leather, Dark Bamboo Trim| Sport Automatic with Paddles | Sport Package
2008 BMW M5 (gone, but not forgotten) Space Gray, Silverstone Merino Leather, Madeira Walnut Wood Trim | 7speed SMG | All options
Mods: Corsa Exhaust | RPI Ram Air Intake | H&R Sport Springs | Rogue Engineering 12mm spacers