Mazda Repair: 92 Miata Rear end noise, rear bearings, pot hole

Question
My 92 Miata is so noisy at highway speeds you can't even hear the wind!  I had the rear bearings replaced and thought it helped for a while, but not like I would have expected.  With the top up it is even louder.  Should I have the differential looked at? Replace the bearings again?  Buy ear plugs?

Answer
Proper diagnosis with the car on a lift and a sonic stethoscope should pin point the source of the noise, anything else is just guess work that will cost you needlessly.  

Rear hub bearings can fail especially with impact related trauma such as curb, hard pot hole impacts or possibly a wheel having been hit in a collision.  They are quite reliable otherwise.  When they do begin to fail the noise tends to increase as the affected wheel is loaded and unloaded as the car is oscillated on smooth level roadway.

If the car has been driven in water also check the drive shaft u-joints for run out, the joints are not serviceable.  

The pinion bearing on 90-93 models though reliable in normal use has proven not to be long term capable of hard abuse such as teenage drivers, auto cross racing and similar.  Pinion noise tends to be constant regardless of oscillation of the car and may increase and decrease with acceleration or deceleration as the pinion thrust moves it forward and backward in the carrier.  Overhaul is optional however, if a low mile replacement is available it is the more practical quick solution.  I would retain the original and overhaul it for future use or sale.  R&R labor should be about 3.5 hours.

Ear plugs remain optional.

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