wheel bearing do tend to wear faster when you put bigger wheels on due to the extra strain they put on the hubs....or so im told.....packing my hub with grease didnt solve my problem and the bearings looked like new so i changed them anyway and the problem was solved. have you tried putting it on full lock while not moving and checking the clearance between the tyres and wheel arches as it may be rubbing? good luck with the ~MOt~ Lets hope its not an expensive one.