Very, very common problem with the 04+ 545/550/745/750/X5's. See it all the time after idling for some time.
Different mechanics will tell you different things, I've done some reading on it and you can drive the car this way should you just want to run it into the ground, but repairs are costly and in many cases, the issue returns.
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--'00 BMW ///M5 .:Jet BLK/BLK:.Individual-Style 65 Wheels; 9.5" Square Set up;275 Tires;Spacers;Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'07 BMW M5/6.:Indianapolis Red
'88 BMW 635CSi
'81 MB 500 SEC Koenig