Well as I've already mentioned it is due to an air bubble in the power steering, the clunk or popping sound is the sound of the air being released into the chamber. Mine does exactly the same. You're wellcome to check with the BMW technicians at your dealership but I've been told that it's not curable. At first I thought it was an anti-roll bar link that was loose but apparently this is quite a commone issue with the latest version X5s and it's down to air pockets being compressed and then suddenly released.

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