1st thing is to make sure you don't have a mix of old and new gears, if poss replace all the gears for new ones including the top shaft. The other reason you could be getting some noise could be due to the bearings you are using, if the bearings have play in them, then due to the tight tolerances in the gearbox this maybe the cause, try some high quality bearings.
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Jonathan | Atomic-Carbon
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