my 1999 olds 88 has a wuh wuh wuh sound starting at 25 mph and disappears at about 30 mph, it is coming from the right side, front or back, i cannot tell.
the mechanic says it probably is a rear wheel bearing, but there is no play in any of the wheels, front or back. this is a puzzle. any ideas?
There doesn’t need to be play in the bearing for it to on its way out.
Sound travels/radiates and it’s always hard to tell from the driver’s seat where a sound originates.
The car needs to be up on a shop hoist and test spins used to help locate the sound.
My 91 Explorer had a sound. Took it to the shop and told them I thought a u-joint was bad.
The mechanic replaced the rear axle outer pinion bearing ! 
Could be a bearing or an oddly worn tire tread. Take a close look at the tire tread.
One way that can detect a failing bearing is to jack the corner up, remove the tire, and trun the hub by hand. You can often feel roughness that isn’t obvious with the wheel on because the wheel/tire mass “damps” the vibrations.
Be aure when you do this that the tranny is in neutral and the vehicle secure from rolling. If you have one wheel up on a “live” axle and the tranny isn’t in neutral, the opposing wheel may turn the opposite way when tiy turn the hub. You don;t want to roll the vehicle off the jackstand. The best way to avoid doing so is to jack up the opposing corner too and let the wheel turn.