I had the identical problem with my 2011 X6, 3.5. On the first attempt to fix the problem the dealer removed the springs, lubed the spring pads and all was quiet for about a month. I drive this car infrequently and it didn't take many miles for the sound to return. The tech at the dealership could hear it when I drove the car over the entrance ramp into the service area.
They drove the car and diagnosed the problem as "defective" upper and lower spring pad mounts. New mounts were ordered, installed and I am happy to report no noise 8 months later. Good luck.