I have the exact same problem in my 2003 330i. The clicking sound is completely random, and I hear it while the car is moving as well as when it is stationary. I also don't have anything in the back seat or the trunk. I thought that maybe the clicking was coming from the box on the roof where the alarm motion sensor is. Then I thought it could be a seatbelt buckle that was rattling around. I don't hear it, though, when someone is sitting in the back seat. If anyone can figure out what the hell the noise is, it would be greatly appreciated.
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