Whining Noise When Letting Off The Gas

I called Rearend Specialist today and they had me bring the car in for a test drive. Immediatly the mechanic said wow thats loud and at all speeds.. They took the cover off and checked it out and found nothing wrong with the backlash. The gears had a perfect pattern at there contact point. he then determined that the Sierra brand 321 gears they sold me do to their cut was extra loud and was amplified do to the fact it was a fastback car..They did not want to put another set of Sierra brand gears in my car thinking they would be just as loud. After searching his stock of gears he found a set of 373 OEM gears that he would install if I was not concerned about freeway RPM..I had called while they were installing the gears the first time asking if they could install 355 thinking i went to conservative with the 321..But it was to late because they had already finished the rear end. Anyways i was happy with going to 373. They installed the new gears while i waited. We went for a test drive No more gear noise.. Just the sound of the flowmasters.. I was very impressed on how Rearend specialist had taken care of the problem. They could have easily of tried to weasle out of fixing my rearend .....Thanks again for all the advice...