If its not the sensor like mine then the PDC brain has gone on the fritz. Just had mine replaced as all sensors checked out and had the continous beep with flashing light. Took them nearly 2 hours to set the system back up again so make sure they do it right!

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Currently 2010 X6 35i, Premium, Technology, Sport.
2004 Lotus Esprit - Final Edition
Former '04 X5 4.4i, Highland Green, Nav, DSP, PDC, Heated FR & R Seats, Comfort Seats, Rear Load Floor, Premium & Sport Package, IceLink Plus, Bel RX65, Tow Hitch, Sirius Satellite Radio.
Former '03 X5 3.0, Nav, Premium, Rear climate. (Stolen)
Former '01 X5 3.0, Nav, Premium, Rear Climate (Lemoned)