Rear End Problems (whisteling!!!) HELP!!!

[barnett18]

3 Months ago I decided to take out the stock gears out of my 99 mustang GT (auto), i bought a set of yukon 4.10's and had them installed at a reputable transmission shop. One day one of my new gears the rear end whistled very loud on the decel. ( it only whistles while moving at 35mph and over, with your foot off the gas). The transmission shop decided to reset the gears and to know avail, the same problem. I turned the yukons in on a factory defect and got a set of ford racing 4.10's. And yet, they are still whistling. And it is loud! Sounds like a dang jet landing beside me. Does anyone know whats causing this? Also, I bought the complete bearing and seal rebuild kit for the rear end the first time i put the 4.10's in. Some please help, the labor ford all this rear end troubleshooting is costing alot!