Rear wheel bearing sounds can be varied, at about 55mpj or so, by swaying the car side to side on the highway. Not a solid sound, but more cyclic in nature. Kind of like a lower-pitched, light "whomp-whomp" sound. As the bearing is loaded and unloaded, the sound intensity will be louder and quieter as this happens on the failing bearing.
Might put the car on a drive-on lift, then elevate the rear axle to maintain ride height, as the car is carefully put into gear and the rear wheels turn to hopefully duplicate/locate the sound.
CBODY67