I went through exactly the same thing a few months ago, I tried all sorts and kept thinking I`d cured it until the next time the long bleep came.

In the end I screwed down the number plates (license plates) rear and front and that sorted it. It seems the license plate was causing the sensors to give a fault somehow (prob obstructing one of the ultrasonic beams?). Anyway it`s been fine for months now.

Just check all the sensors, make sure there`s no dirt stuck in the little grooves, make sure nothing is loose around them, make sure your license plates arent sticking out a few mm more than they need to etc. If not, then you`ll have to take it back to the stealer and tell them it was fine before they got their hands on it.

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X5 3.0D 2003
Mods:
Webasto telestart remote kit
Aluminium running boards
Tv in motion & Aux input lead, Ice>ink to Ipod 3g.
DVD rear headrest screens and dvd sytem integrated in to main tv system via aux lead (watch tv or dvd in front and back)
Homelink Gate Opener, Chrome Grille Kit, chrome tailgate handle, Chrome Guage Rings, Tunit V-CR 086
Standard:
Bluetooth, comms pack, sat nav, sports pack, sports seats, dimming mirror, electric folding mirrors, memory seats, heated seats.