Most likely a duff sensor. I had the same problem on my previous 528i. If you put it in reverse key position 2 (engine off) then go around the back (stand to the side clear of the sensors). Put your hand 2 feet away from the sensors one at a time - a good sensor should cause the PDC to beep intermitently as normal as you move toward and away from the sensor. The sensor that does not respond (constant tone only) is the bad one. They are not difficult to remove - look under the rear lower trim or remove the tow hitch cover and you will see how they fit. The sensor fits into a sleeve that has a retaining catch - gently pry the catch and draw the sensor back towards the car.

I know the constant tone is annoying when the system is active but the other sensors should still work when in range of an obstacle so the system is still useful. You will find these sensors cheap enough on eBay regularly but check the part numbers before buying - there are a load of different types. Alternatively try a breakers yard. Sensors from other BMW models may be the same. I was lucky enough to get a set of four for the E39 for about $60 on eBay (from an E38) so I ended up with 3 spares. (these are an older 2 pin connection type - not likely to be any use to you).

In the meantime you can locate the sounder in the rear compartment and muffle it or unplug it if you must. but I encourage you to replace the dodgy sensor (you'll kick yourself if you scuff that bumper for the lack of a working PDC system).

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Cheers, Brian

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2001 X5 4.4i Sport
Black Sapphire, Hi Gloss Shadowline, Full Privacy Glass.
Too Cool for Jack Bauer