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-1 of 1 - Style 65 Wheels; 9.5" Square Set up;275 Tires;10mm/3mm Spacers; Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'04 Volvo S60R/6spd.:Nautical Blue/Gobi&Black
'00 BMW ///M5 .:Avus Blue/Grey/Black:. 1 of 8 SOLD
'88 BMW 635CSi Blue/Tan Sport
'81 MB 500 SEC Euro Koenig