Folks,
I have a 96 Disco, Daily driver & weekend wheeler.
I installed some long awaited 4:11 gears in my diffs (reverse cut in the front). Love the new found power/acceleration, however, the rear diff develops a higher pitch whine at about 70 mph. The whine is not there on acceleration or coast at that speed - the whine is only at a "sweet spot" cruising. I also hear it to a lesser degree at 35 mph same deal not on acceleration or coast. In most circumstances the diff is quiet. I don't mind the sound it is not that loud but you do hear it clearly when it is whining - - the radio can mask the noise. I really don't want to risk or kill a new set of gears - - I waited too long to get them. I changed gear oil at 500 miles very little break in shavings seen then. I will drain and add new gear oil again in about another week the gears will be at 3000 miles at that time. FYI: Installed new bearings on the ARB locker's. New bearings on the pinion. I checked the runout on the ring gear and that was within spec. Set the pre-load as prescribed, reused same pinion shims then patterned the gears and the pattern looked good to me. I set the backlash @ .0007 on the rear - - front is set @ .0009. The front diff is quiet - - Any ideas? Could the backlash slightly tighter in the rear cause this whine? Would this type of noise suggest I missed something on set-up like pinion depth? I am thinking about pulling the diff and re-setting the backlash on the rear maybe re-check the pattern - - just to double check. I am not sure that re-setting the backlash would help the noise - - would it? Do 4:11's tend to whine and this could be considered acceptable/normal? It is a Rover and I can live with the whine - - just don't want to do any damage to the gears. BTW it doesn't sound like a bearing to me - - no groan or metal grinding and the whine doesn't come and go with turning or other forces only whines cruising. Drive shafts are in good shape - not a u-joint issue and no vibrations.
Thoughts or other things to check would be welcomed - - anyone experience this prior to going large and installing your 60's? :homer: Thanks, Paul

Is the mating pattern consistent? If the ring isn't really flat on the the diff, the ring can distort enough to cause the pinion to mate inconsistently, which can get things whinin' at certain speeds. You might need to do some sanding/oilstone on the mating surface to get a really flat mount to the diff.

I'd be very suprised if you got the pinion height right by just re-using the original shims. I've set up a shit pile of Rover diffs and have yet to have that be the case. Generally a good start point, but not the end point. The pattern should tell you this though, and a good pattern is more the result of correct pinion height than backlash.

Thanks for both of the responses.
Sounds like the suggestion would be to pull the diff and recheck everything. Both of you feel that a whine at specific speed crusing IS a sign of improper set-up in the rear diff. This is where I was headed - - I do appreciate the experience and advise. BTW, I did put a stone on the back of the ring gear and the run out was within spec. but I will recheck this while I am there. Thanks for the thoughts.
Q:
Would improper pinion depth either too deep or too shallow cause the whine or in your experience? Or would a whine only develop if he pinion is too deep? Sounded like you are sure I missed resd the pattern.