The DICE could be picking up interference from other devices in the vehicle. You can try relocating the wire to see if the noise is diminished or if you cannot find the source of the noise - a ground loop isolator must be used.
Some BMWs are more sensitive to engine noise than others. Here is a link to the version we offer on the website:
12 Amp Power Noise Filter
http://www.europeanautosource.com/pr...roducts_id=309
Let me know if you have any further questions, or a radio harness would be an alternate solution.