Just to toss a bit more clarity into the mix;
Gears are one of the most difficult systems of mechanical engineering.
Purpose, environment, shock, expansion, duty cycle all calculate into the mix. Tooth profile, materials, loads, lubricants.
Heavily loaded gears are always steel, with heavy, high-additive lubricants. Lesser loads can be handled by lesser materials; but Aluminum is at best a poor bet in any case. High-speed gears subject to high heat demand specific lubricants; think your rear drive unit with Red Line heavy, etc.
Well your engine doesn't have Red Line heavy. Your engine has motor oil, which is *not* a very good gear lube. One solution is to use very high quality, ground-profile gear teeth that don't clash. What you hear in your straight-cut gearbox is actually each tooth bashing into the next one as the gears rotate. That's a lot of abuse. To eliminate the noise, the teeth have to be finish-ground rather than hobbed (cut) to have the correct profile and finish, and the clearance needs to be within a very specific range. Aluminum is not out of the question here, with the caveat that gears have a minimum threshold for load capacity, obviously, and metals have the very odd property of disliking mating gears with the same hardness. One gear must always be harder than the other, unless both are hard enough to have zero deflection and adequate lubrication to keep them apart. So by the time you create an aluminum gear of high enough quality and finish, and mate it to a steel gear of sufficient hardness, you may as well just go with steel anyway, which is what Joe Caruso has done.
I would also wager that if you could watch the cam gear on our engine with a strobe light, you'd see it flopping all over in a wobble as the camshaft deflects, which leads to edge loading the gears, multiplying the problem. I wonder if Joe has a tiny bit of camber built into the teeth to prevent that edge loading.
Pay no attention to the man behind the curtain, I'm tired and rambling.