KM-145 5th gear whining/grinding common?

I just mostly finished installing a new used transmission in my 87 4x4 and decided to try it out on the road behind my house in low range so i could go through all the gears without speeding. 1-4 seemed to work great however 5th made a pretty loud whining/grinding noise which leads me to the question of whether this is a common issue with these transmissions or just 5 speeds in general and what the cause normally is? Im assuming its common because a parts truck i used to have had a similar issue and the transmission that failed in my truck had a noisy 5th gear too. Is this most likely caused by bearings or bad gears? im thinking about rebuilding my original 5 speed and using my current one till it fails or i finish the original one but I would like to get an idea before i tear it apart whether i will just need a rebuild kit with syncros and bearings or do i need to be looking for a new 5th gear too? I haven't flushed it with a cup of diesel like geezer recommended but i will soon. sorry if this has been asked before but I am new to transmissions and i haven't noticed any threads discussing this issue.

IDT if it's common but I did help a friend pull a RWD 5 speed out of a Gen 2 at a yard, only to find it had a blown 5th gear...