Sounds like the starter to me. I had this issue with my 2003 540i and at first it was intermittent then it will get progressively worse. It has to do with the starter not being able to disengage once the car is running. From what I remember reading on another forum you want to get it fixed quickly to avoid any additional damage. Some people did talk about replacing something (spring?) on the starter but for me it was just easier to replace it and be done with it.

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2006 X5 4.8is