Probably too much backlash in the ring and pinion,but it could be lots of other things--first thing to do is to pull the driveshaft off the rear end yoke and see if the big nut has come loose,if it did,this will screw up the pinion backlash,(usually the pinion seal gets trashed when this happens,and leaks )if you tighten it up right away,it may survive without damage.This is not always easy to do in your driveway,you have to hold the yoke still with a big pipe wrench,and use a breaker bar and socket to re-tourque it--some folks use the impact wrench,but if you tighten it too much,youll crush the crush sleeve inside the rear end too much,and the gears will wear out rapidly.You might be wise to take it to a gas station or repair shop that has experience in doing this kind of work--or start saving for a new rear end and stay home a lot! Or maybe you may luck out and find a bad wheel bearing or other cause for the noise--good luck! /forums/images/graemlins/crazy.gif