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First Helical gears are much stronger then spur (straight cut) gears in  fact inside  a helical gear is a spur (straight cut) gear sitting at a angle The difference in both tooth elements and gear elements is the difference between the Real Diametral Pitch and the Normal Diametral pitch. Normal being a spur gear real is once the angle of helix alters.  Oh since its metrics its probably Modal Pitch but the same gear theory. All gears have a calculated back lash which is why to start the clearance in the bottom of the tooth is proportional to the tooth size as with calculated back lash. this is measured on the Pitch circle of the gear. with the large back lash eliminator on the main drive from engine to clutch the fact that the gears are floating on a film of oil except for a couple permanent ones like first and second on the input shaft and fifth gear on the output shaft  I would say if there is vibration in there she lost oil and one of the bushings the gears ride on is galled up.  I would say highly unlikely and if so you have major oil problems elsewhere like up at the cams. No I bet not.