Fighting same exact issue and am in the middle of a lengthy process of elimination. Replaced diff fluid last weekend to see if it made a difference. None. Not thinking ujoint since the car is super smooth until that point (45-55mph) and then super smooth again after the grinding stops. Mildly interesting - - I put new control arms on the car 3 weeks back and the tone and volume of the grinding sound is worse. Thinking the tighter geometry of the rear axle changed it.

 

Update on this - - spoke with a local garage at length to determine the next item to fix on the "process of elimination" list. He says this is most likely a pinion gear. He also said it is not bearing related since it starts at 40-45 and (most importantly) goes away at 50-55. Going to use the checkbook wrench on this and will post back with results...

 

what brand name gears did you use?

 

i put a richmond 3.73 with a locker but also now it is making that noise all the time and when you come to a stop you can here the gears start slowing down like a whining sound its really making my mad

 

richmond gears whine. gona have to get used to it.

 

or change them i dont think they where set right i think the shims are wrong taking it to the shop wensday also having problems out of my other 97 z28 trans is acting up it only has 66000 miles on it and when you put it in gear its fine for a mintue then it dont want to move if i shut it off and start it it will move again for a minute then wont move an any gear