Chime? Do you mean you get a long beep when you put it in to reverse and then nothing?
If so this means one of the sensors isnt working anymore, happened to me a few months back.
Put ignition on, put car in reverse, put your ear to each sensor and you'll hear them clicking. The one which isnt clicking is the dud. I bought one on Ebay for £27 and fitted it myself in about 20 minutes. BMW want over £100 for the sensor and £50 or so quid to fit!!!!!
