Feroza gearbox


Hi all,
I have just bought a 1994 Daihatsu Feroza (Australian) and i have noticed a very slight and brief grinding feeling as i change from 2nd to 3rd gear, i can't hear it just feel it in my hand on the shifter.
I have read through alot of other posts about sportrak gearbox's doing similar things, does mine sound like it could be a problem with that main shaft nut coming lose as well??
All other gears a fine and cannot hear any noises coming from the box.
I am hoping it could be just a clutch adjustment!, but if it was it would effect all gears not just going into 3rd, right?

Thanks for your help