I know for sure that it can't be the bearings because I cleaned them and inspected them extremely thoroughly during my rebuilt and all were in tip top shape after I was done.
The diff balls and rings: they looked ok to me but I must admit I didn't spend a great deal of time examining them.
If they were anything less than perfect would they be causing that noise?
Anyone who heard my audio file and could comment on the particular rattle noise? I'm hoping someone experienced could make a good guess from the noise.

Finally, how can we explain that it ran very smoothly right after the rebuilt for about 5mn? I thought that maybe the grease got pushed back on the inside walls of the tranny after the gears spun so many times and the gears didn't have enough grease left on them to stay smooth and silent? How much exactly am I supposed to put in there? I've heard some people "filling up their tranny box", is that what's recommended?

BTW, this is an Evader Pro BX stock tranny, top shaft is steel, idler and diff gear are pastic.