OK, I just had my 9" rebuilt and regeared to 4.88's. I've put about 300 miles on it and its got a SERIOUS whine that starts at about 45 mph and gets totally unnerving at 75 and above.

When your traveling at highway speed the whine changes pitch if you accellerate or let of accelleration and stops if you just push the clutch in or put it in neutral.

I replaced the axle bearings so I don't think that's the problem.

Input from any axle pros appreciated!

thanks.

 

Sound's like you don't have the pattern right.

 

When changing from a stock 9" diff. ratio to a more reducted ratio like 3.89, 4.11, 4.56 and numerically higher, some special attention needs to be done to the inside casting of the 9" carrier (the housing that the ring & pinion are set-up in).

The straddle that supports the tail bearing on the pinion needs to be tricked, or finish ground to accomodiate the thicker ring gear. When I have re-ratioed to 3.89 and above, I also make sure that all internal seams are blended and smoothed. Afterwards, carrier housing is agained cleaned, dried and internally sealed with a good quality red oxide primer.

The noise you are describe could be an incorrect contact pattern of the ring gear to pinion mesh. Another possibility: internal casting junk, flaking off, or being milled off by the thicker ring gear and contaminating the pinion bearings. I would get to the root of this problem soon, otherwise, you will have a junk carrier and a completely contaminated axle housing and axle bearings.