We had our rear diff replaced under warranty. Eventually found a dealer that felt the same as we did. They traced the howl to the pinion bearing. With the new diff I would say the noise is much less, but its not completely gone. They (techs) say that some quiet whine is normal from the xdrive bmws, especially during deceleration. Will see how it goes, I think i'm hearing it get worse after a few hundred miles on the new one but maybe not, unfortunately i'm now totally over senstive to the sound.