Put the Tcase in N and see what it does.
It almost sounds like the trans now. Could be a countershaft bearing?
Did you say this was in a zuki? Does this have the devoiced tcase?
check middle driveshaft and flanges, if there is one.
Also if it does have a center shaft you could remove it and see if there is still a noise and spin the Tcase to feel if it is rough.
Don't forget the screwdriver idea to check for the noise. (loong screwdriver)
