Ok Pedro, there's no whine, but in your first post all we had was a "gawdawfull racket', which could have been clunks, clatters, rattle and hum or a whine.... which I'll say more about later...
In my previous post, the noise I referred to was actually what sounded like your description
the noise in third is a clacking like teeth aren't meshing.
, but if this was your original noise, my remedy is n/a anyway, but worth keeping in mind on reassembly...
If you haven't already dismembered the box, check the shaft end-floats first, and if there's a gasket in the joint, take note of its thickness, as that affects the layshaft float (obvious).
I suggest taking photos at each stage for your own reference...
The obvious check for bent shaft is roll it on a flat surface..?
I can't think much else that hasn't been said until you update us.....enjoy the sangria...!
Maybe not related to your issue Pedro, but just a thought on Trevs' post;-
I expect any clutch drag, and mismatched revs will help the selector forks wear quicker ...?
Selector fork scuffing ( and thus wearing ) will cause a whine in that gear only and oft it will be different depending upon weather you have up shifted or down shifted into that gear.