If it is something wearing, or a bearing braking up in the box, then the
magnet in the oil drain plug (magnets in the plug are pretty universally
fitted) should be thick with fuzz and bits of anything that is braking up.
Can't remember if the transmission plug has a magnet, the engine plug
should have one.
You can sometime diagnose from any of the larger bits what, if anything,
is disintegrating.

Tip - get a rubber bung that fits the drain plug holes, the push the bung
in as you take the plug out, that way you don't have to drain and refill
the oil. Further tip transmission and engine oil after running are HOT
particularly running over your hand,  and down your arm, it's usually
cool by the time it gets to your turn ups. Post again if you find anything
interesting on the plug. It's quite profitable replacing gearboxes. I'd do
(or get a local well recommended self employed mechanic to do them)
the above checks which should only take abut 15 mins, and give a
second opinion. Sure it's not the clutch thrust race, does the noise
alter when you put the clutch down? Putting the clutch down disconnects
the transmission and also slightly alters the pressure on the crankshaft.
So any change in the noise can give clues.