is it only make the sound in gear or when you let off on the clutch? if it makes the sound in netrual with the clutch disengaged then you have a bad throw out bearing, mine as well do the pilot bearing as well. if it makes the noise in gear, if makes the noise in all the gears then probably your input shaft bearing is bad, but if it makes the noise in only one gear then its a syncro in that gear or some other bearing. if its a bad input shaft bear or a syncro then throw the tranny away and get a good used one, not worth rebuilding them unless you plan on making some real power.