Yeah, realoem only shows the crank pulley as fitting e70 x5s. It must be due to the accessory drive. Would you happen to know the other thread on xo that had a similar issue? I haven't been able to dig up much other than "it's not tensioned enough", there are bad bearings, or the belt is contaminated. The latter is a possibility but I don't think that's it considering it started on the first drive and all the belts are new.