I have a 2001 325Xi with about 59K on it. The rear end hums above 50 mph and only gets worse above that. It seems to be coming form the center of the car. The rear wheels are new and the tires are new. The brakes were changed and did not change anything. It is not a tire balance issue, I have had them checked and rotated. It seems to be a bearing noise, and does not change even when cornering. Anyone have any experience with this sort of issue?