My 55,000 mile 2000 Safari van has started making a sort of whining/humming noise from the rear end when Im driving about 30-45/45 mph. The noise is not constant nor is it everytime Im at that speed. The dealer mentioned something about the differential but could not get a noise duplication but they neglected to tell the tester of the mph range and told him it was a " roar" which its not so they didnt investigate further and sent me off. Im just concerned the rear end is going to fall out or lock up going down the road one day. Ideas?