I just had the same problem with my trans. It's hard to describe the noise but it was a kind of rattle/chatter kind of sound. Mostly noticable in 5th at highway speed. It would go away when accelerating and come back when unloaded and go away again when decelerating.
It drove me nuts for months. Oil was clean, no metal on magnet in trans or T-case. Shifting was ok but kind of "notchy" feeling. I thought the syncros weren't broken in.
I was ready to pull it when I read the debates over GL4 vs GL5 gear oil. I was running 75w90 GL5.
Some articles say it is too slick for the syncros to slow down and it has a higher sulfer content that could shorten synco life over time, etc. Other articles say it is perfectly fine running GL5.
I took a chance and got some 75w90 GL4 to try before I pulled the trans. All I could find locally was Redline. The trans now shifts super smooth and the noise is completely gone. I wouldn't have believed it if I didn't try it myself.
I'm sure people are going to chime in and say BULL, but try it yourself and see. Let us know if it works for you.
