this could be the input shaft bearnings noisy in neutral but noise dissapears when clutch is despressed!
or the output shaft as in the driveshaft, you will tell when the output shaft ones are done because
you will get oil leaking out of them and there will be play when you move the shaft!
if the input shaft ones are noisy thats the shaft spinning the cogs then it may last until the car falls apart...
where as if the output shaft is at fault they wont last long few months max and eventuly the differential will have enough movement to bash the inside of the gearbox casing making one hell of a noise when your driving!
this happened to me this is how i know

the noise when you took your foot off the throttle in gear it sounded like the whole exhaust had come off and was dragging under the car it was that loud as in metal grinding!
i remember people looking at the car wondering why it was making that noise, yet on the outside you couldnt tell anything was wrong!
