Mine does this too. When my friend plugged his ipod in, it did it fairly consistently. When I use my android phone, it does it but only randomly. It seems to happen more often when the phone is plugged into a power source. I can only guess that the phone is sending some sort of signal up the aux cable that is confusing the car's radio. Maybe the fact that the phone's aux connector can use a mic as well, I don't know. I haven't found a solution. I thought it might be the connector on the back of the radio coming loose, but I checked and it was totally fine.

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2003 X5 3.0i, 150k miles