Corolla 1.4 d4d 3rd and 4th gear whine

[Stepheeen]

Hello guys! 

I've been searching the internet for similar issues, but nothing concludent. So my 2008 corolla E140 1.4 d4d 5 speed manual transmission has a slight whining when accelerating mostly in 3rd and 4th gear but more noticeable in 3rd that i think is coming from the gearbox. I would describe the sound as a high pitched whine (like a jet plane ready for takeoff) that is present only when in gear and only when the tranny is under load and with slight acceleration, but dissapears over 2700 revs or so (or just get covered by engine sound). 

The transmission fluid was checked and replaced, it had the normal wear brown color, but I didn t find shavings in it. The issue persisted even with the new fluid in. The car has 240000 kms on the clock, and the fluid had never been replaced until now as far as I know from previous owner. 

With that being said, what the problem could be? Gearbox bearings, gear wear or syncros? I must add the gears are shifting smoothly, only reverse gets a bit of resistance from time to time, no clunks, grinds, clips, chirps. Just high pitched whine under load and slight acceleration, only in gear.