850i 91 strange sound

Hi,

I have really a strange thing going on with the 8. There is a strange stomp/sound with regular intervals that you can hear when I apply the brake. You can hear it the best when I'm standing stationairy. When I relaese the brake the sound doesn't occur. I've made a video, hoping someone recognizes this sound and knowes the answer... because it drives me crazy. (I also feel the stump in de brakepedal)

http://www.youtube.com/watch?v=H3OHCCkkC68&feature=youtu.be