I bought a used stock 10 bolt with Motive 4.10 gears already installed. The gears were very quiet when I first bought them but eventually got really bad. (this was with a shot clutch and bald comp TA's). I had the rear end taken apart and had all new bearings and races installed, new mobile one synth fluid with GM additive, and a solid pinion spacer installed. Now the gears seem pretty quiet under acceleration and mildly quiet under a constant speed cruise, but when I am decelerating they whine like crazy. The guy that did them said they didnt have the greatest wear pattern but he said he set them to wear properly. Does anyone know what causes a whine under decel only? And will this ever go away? The next time that rear cover opens up im going back to 3.42's :-)
Any help is appreciated.
Thanks

 

Could be preload - Backlash? Not sure. Could be how they were worn in while new...

 

When slowing down the pressure is on the back of the gear teeth instead of the front. Sometimes the backlash can be set correctly but the pattern can be off. You can pull the differential cover off and check the pattern with some marking paint which you can get at any auto store.

 

Unfortunately, once they start whining, it can only get progressively worse. So no, you cannot stop them anymore.

Most gears that whine do it under deceleration. My guess is they weren't properly installed by whoever did them before you bought the rear.