If a picture is worth a thousand words then here’s 1 million:
https://youtube.com/shorts/UcHbYZo9tEA?si=gZWtxyemQhUTOR4Op


I can feel it in the stick as well (yeah, yeah, that’s what she said).

 

"Unavailable video".

 

I’m so stupid. I’ve corrected the link and also noticed my message was bungled.
It’s now no wonder that I got no replies :)

 

You don't have rattles in any other gears, especially in reverse? I don't know what exactly would cause a rattle like that, especially since the transmission otherwise seems to be working fine, but it looks like when the 5th-Reverse synchro engages with the gear for 5th on the countershaft, then and only then you're getting the noise. Since the synchro seems to be working fine, maybe you just have a bad countershaft and/or output shaft 5th gear (#14 and #12 respectively). You potentially could pull out everything shown in this diagram with the tranny still mounted to the engine. What's unknown to me is how difficult it might be getting the linkage that operates the shift fork matched-up with the fork. That's a three hand job with the tranny out and the top cover off.