If the tranny fluid level is okay try changing the fluids, if you haven't in the last 100,000 miles I use to run straight ATF in my non O.D. 833, they never made noise unless there was a phisciycal(SP?) fault. I did a trans for a freind who trans, non O.D. 1971 A body, made noise in all ears except 4th gear(direct). The main shaft(output) had been ate up where the needles bearings ride on from inside the input shaft I had it welded up with a really hard welding rod and turn back down to stock size by a old timer who had rebuilt H.D. truck trans years ago back when labor was cheaper than parts where