13 speed rattling, 5th position not engaging

Loose u-joints wouldn't explain losing a gear or rattling at idle

 

When you say NEW transmission.. do you mean Brand new out of the crate ?
Or do you mean new rebuilt transmission ?

 

It could just be your tower is getting worn

 

When my 13 went bad, it did that. Lot of noise. Didn’t pop out of gear though. Dropped a gear, less noise, ran it in direct, for about 350 miles. Diagnosis was left (or right) ? bank bearing failed. Easily seen with bottom inspection plate off. The bearings sit highest in the trans on that side. The front bearing that holds the bank of gears was shot, the whole bank of gears was loose.I’ve been told it’s the weak point, most common failure in a 13.

 

If its a double over 13 speed I suspect you have a thrust washer failure on the
main shaft.

 

Have you checked your splitter valve .
Thats what happened to me also.
I would hear a zinning buzz sound when i went from 4 to 5 and it would pop out of 5th sometimes.
The splitter was leaking air going from low to high range.