a lot of people have the same problem when changing to a new clutch and T.O. bearing. I've heard that it is most likely one of the bearings inside the tranny like an input shaft bearing or something. I have the same problem after changing out my clutch/flywheel TO bearing etc. I havent fixed it yet though cuz i have no $$ but i hear that you dont want to drive on it too much cuz if the bearing blows it can do more damage to the tranny.