What is the coolant level in the reservoir when the engine is cold? Is the end of the red float aligned with the upper edge of the filler opening? If not, you could be experiencing exactly what I went through with my X5 2005 4.4- over-pressurized cooling system due to coolant overfill. I replaced reservoir twice due to cracks causing leaks and coolant leaking from under the car when it sat for 3 or more days. Finally discovered I was overfilling the coolant (red float all the way up). Simply correcting the level fixed the leak and no more cracking reservoir.