Originally Posted by Cutlass327
Being 4th is direct drive, where input and output shafts are locked together, I would almost have to say it is in the bearings for the cluster shaft. I can't remember the technical name for the shaft...
Cutlass327 is right. The countershaft cluster gear is used in 1,2,3 and reverse but not in 4th because it's locked directly to the input shaft when in fourth. If it's noisy in all gears besides fourth, it would probably be the cluster gear bearings. Sounds like tranny teardown time.