Need help diagnosing a "clunk" when braking quickly

[pgolimowski]

I have an 08 x5 4.8 with about 45k miles. Only have had this car for a few months so I'm not sure how long this has been going on. When I push the brakes slowly, it brakes normally. If I push them quickly, but not necessarily hard, it makes a loud clunking noise that can be heard and felt in the pedal. Before my warranty ran out I had dropped it off for service and had them check the brakes and they said they were normal. This is about my 10th BMW and this can't be normal. Any ideas of what to check would be appreciated.
Thanks
Patrick