Could be the Clutch bearing, it sits on the shaft from the Gearbox and is able to slide up and down the shaft.
If this wears, it normally only rattles when it has no pressure on it, i.e. when the clutch is fully up.
Not too sure if it would make the noise when it's in gear and the clutch up, but then you'll be going along a quite a pace and might not be able to hear it.
If this is the case, then you should only have to press the clutch down slightly, to put some strain on the bearing, it not then it could be something internal to the gearbox.
I hope it's not the latter..
