The manual box in my 300zx TT has allways whined a bit but over the last few weeks i've noticed this weird sort of squeeky whine, over and above the normal gearbox noise, which is inconsistent in duration and only seems to be apparent when the car is cold. and commonly at around 1500-2000 rpm

 

I cannot seem to pinpoint the exact source of the noise but it appears to be around the box area.

 

If, when i hear the noise, i maintain the throttle position and dip the clutch--the noise continues but dies with the car speed.

 

Any ideas!!

what gears does it happen in. If it is second and fourth, it could be the little bearing under the selector needs replacing, or the rubber boot has come loose.

No big hassle or expense on both accounts, just difficult to fit i think

Could be the Differential. Try changing the oil in the gear box and diff and see how you go.