A possible source of noise on the left-hand side of the ACE 1330 engines maybe the straight cut (spur) gears used on the cam reduction drive train. Click on the photo to enlarge. The straight cut gears do not put an axial load (thrust load) on either gear but they produce noise. Helical gears would have been much quieter but may have required a thrust bearing surface. BRP either choose straight cuts gears as a cost reduction measure, or an enginering solutions of some sort. Here is an article that discusses straight cut gears (spur gears) vs helical cut gears.

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