It may behoove you to pull it apart now. When does it make noise? For instance, my D250 was taken off the road before I bought it because it was making a racket in 5th, and a little in 4th. Pulled it apart and the 5th gear bearing was fried. $20 repair, no special tools.
We had the rear countershaft bearing go out in another D250, it knocked a hole through the back of the tailpiece. The bearing itself was a cheap and easy repair, a little welding took care of the tailpiece. Couldn't find the seal for the - if I'm remembering right - the front countershaft bearing; a little JBWeld took care of that...
Point being, if you can isolate a bad bearing as the cause, easy fix... if you let things go, they aren't going to get any better. Case in point: my W250. But that's another story...