I had the same issue some time ago, I took the care to the BMW dealer and the issue was slush within the vanus.
This was caused by the use of the incorrect synthetic oil.
As a result, I no longer replace my oil with the Formula 1 synthetic oil, I now buy the Lubro Moly Voll-Synthese Motor Oil - Synthetic 5 Liters - 5W40, I buy it from Bavauto.com.
It's a German made oil and it really makes a difference.
Here is the description:
Endorsed by BMW! Dramatically decreases engine wear and oil consumption while improving mileage and performance. The 5W40 formula increases protection of your BMW's camshafts, valve-train and engine bearings over standard mineral based oils. Please note: Your Vehicle Requires 6.9 Quarts Or 6.5 Liters.