bearing going out. Probably the mainshaft bearing. They are actually one almost anyone could replace with a bearing puller and some common mechanical sense. Just make sure they are the same width to the "T" or you might have some problems since your dimensions would be off in your trans and it would have to be re shimmed. But i do have a transmission in my shed that shifts like butter, has no grinds but 4th gear whines while you are in it so I have a bearing going out in it.