Doing a little more thinking, and a bit of googling, I feel confident in saying the noise probably increases with wear on the gears.
Or, the gear clearance wasn’t as tight from the factory. Austrian koalaity control makes me lean toward the gears just being worn.
As the clearance between the gear teeth opens up, the teeth which once smoothly eased onto each other, now hit.
That makes the annoying noise.
For shits and giggles, swap the clutch bell from the quiet engine onto the loud one. Hopefully those clever Austrian engineers made the clutch gear ever so slightly softer than the final drive gear. In which case, the clutch bell (which is easier to replace) would be the consumable part.
Someone will check me if that’s all crazy talk.