2006 BMW 330 Rear droning noise

I used to have the factory run flat tires and after about 35-40000 miles, I became aware of a droning oscillating noise from the rear of the car. My first instinct was that it may be the tires, so after about 47800 miles, I put new ones on (they were due anyway). Even after new non-run flat tires (used to have Bridgestone Potenza runflats) I still hear the same droning noise from the rear of the car that becomes obvious above 30mph. BMW checked the bearings (claimed to have) and said they are fine. The car is still under warranty and BMW simply attribute the noise to the tires (maybe they are avoiding to fix the problem). What else could be the cause of the problem?
Your help and advice is very much appreciated, thanks.