As has been suggested, I would look at the rear brakes. As you're driving, does applying pressure on the brake pedal have any effect on the noise? How are your rear wheel bearings? If you jack the rear wheels up, can you shake the wheels up and down and side to side? Do you have any body parts rubbing against tires? That could explain why when the mechanic went for a ride with you the sound went away--different loading on springs. Just my thoughts. Stuff like this can drive a guy nuts. <img src="/forums/images/graemlins/confused.gif" alt="" />