I need some help diagnosing a strange noise.  I'm hearing a soft
oscillating rumble with a period of about 2.5 seconds at 70 mph that
appears to be coming from the rear of my MT '86 Subaru powered Westy.
It's not related to the engine since it is there while cruising at a
constant speed or coasting with the clutch disengaged.  Brakes are
smooth and in good shape.  Wheels and tires are also in good shape and
ride smooth with no out of round or balance shake.  Front wheel bearing
were recently repacked with no effect on this noise.  CV joints are well
greased and have no unusual stiffness or slop evident from moving
half-shafts while installed.  Rear wheel bearings were repacked about
40k miles ago.  Vehicle mileage is 290,000 miles.

Any ideas?  Has anyone experienced an actual failure of a rear wheel
bearing, an if so, what were the noises?

Thanks for any information.

Ed