To be honest those gear pinions don`t look too bad at all,no chipping or burring evident but yes,perhaps a little `pick-up` of a harder material on the faces of the teeth,or perhaps they were never manufactured/machined very well to start with.
As Steve Speed says,maybe a bearing had broken up at some time previous to the strip down and not enough care went into removing the debris.
Sometimes there are natural frequencies at play in gearboxes,third gear could indeed just reach such a frequency and the whining is a direct consequence of that.
Unless gearboxes offer too many false neutrals,clunk badly when changing gear or jump out of gear it`s often not worth worrying about them too much,costly business splitting crankcases for no good reason.
Perhaps a poll should be carried out by forum users to ascertain if third gear whining/gearbox noise is common to ALL TRX`s?
That way even high milage or abused/track used bikes will tell the story of `it makes a noise but it remains reliable`.
