i had the same on a mondeo, with a ford it usually comes as a complete unit and cost me about £30, u normally have to remove the central wheel nut and sometimes the brake disk to access it and swap fit the new one. you dont normally need to do both wheels just the one that is humming. a little test would be to grab the top of the tyre tighty and yank back and forth to see if there is any movement, if there is then its likeley to be the bearing.
bruce_uk
January 2008