If it is internal to the transmission, you would be able to find crud in the oil (filings, chips, chunks, etc.). If the noise and rattling is in all gears, then it might be the primary reduction (between the motor and the clutch). Anything you do requires the removal of the motor and transmission from the bike. Even if you just want to remove the cover on the clutch housing to take a look. If you don't have a shop manual that covers the transmission, I wouldn't try to open it up. Also, gaskets or internal parts are not available for the transmission. Again, inspect the oil in the transmission to confirm that the problem is internal to the transmission. If it turns out you need parts, you might try to contact IET to see what they can provide. However, I don't know if anyone in Italy can answer the phone right now.