I will say that IMO anything thinner than 90wt youll hear "things" you wish you didn't, happens when they get low oil, even the 90 wt, they will clang and rattle and shift wierd, thats when its time for fresh 90 wt.
I too would defend Pig, Onur, Steve and anyone else on here. Help, help diagnosing the MYSTERY noise ????? I will offer that my sm420 which has been used, rebuilt, used some more, behind 2 different engines, I've rebuilt it, & my tcase alot all by myself, not speaking down to you just offering...
My driveline will make a noise and vib at certain speeds, the shifter on the 420 will rattle as you described in 3rd gear, did it happen in 3rd when it was making the noise, FWIW it wouldnt hurt to pull the inspection cover on tcase and have a look. My 420 has lots of miles, rebuilt it in 2003...
If they fell out it will eventually cause shifting to second issues and input bearing issues, fwiw, IMO the 420 would eat those up and youll never know, I too had same issue, took front bearing retainer off and input moved forward, tink, tink, tink, what was that, learned how to rebuild a 420.