Get thee to a dealer. I had one go, they just replace it. It is an expensive part, but I assume you are still under warranty.
'05 E90 325i 6sp Man SG 118,032km on the odo (retired)
'06 E90 325i Step SG 121,021km on the odo (retired)
'08 E90 325i LCI Step | Space Grey | Black/Burr Walnut | Adaptive Xenons with HBA | CA | Navi Pro | 17in mixed tyres | 105,437 on the odo