Could be also be caliper piston sticking or a bad rotor even though it doesn't happen all the time. Next time it happens try letting off the brakes and getting back on them rather hard to see if the vibration continues or goes away. Optimum conditions would be that it almost always happens when the brakes are cold or when you have driven 20-30 miles without using the brakes and the braking.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas