I've seen this twice before on 3.0 petrol cars, (once on an x5 vband once on a 330i touring). Both times it was just condenstation so fingers crossed. To be double sure you should get the hydrocarbon test done. It's basically a bottle of fluid that attaches to the header tank when the car is running. If there is any exhaust gas in the water (indicating a head or head gasket failure) then it changes colour. Best of luck, fingers crossed for you!

Hop