I've just finished building the car. Knowing beforehand of the transmission issues, I made sure to build it really carefully, with gears moving freely, and aligned the universal joints correctly. Before doing the bodywork, I attached the wheels and checked that all gears worked. 4th Hi was not smooth and it had a lot of drag, but worked without cracks, so I kept building since I didn't manage to improve it.
Surprisingly, after finishing the model, 4th Hi (the lowest gear) cracks when moving the car forward, but not when moving it backwards. All other gears work perfectly. It seems like the body places some stress on the transmission that wasn't there beforehand. In that gear the transmission seems to work like a spring, storing energy that it ends up suddenly releasing either as a spin of the fake engine, or as cracking gears.
I've checked everything and I have been unable to see which gear is cracking, though it seems it comes from the front part of the transmission.
Any idea to fix it?
EDIT: since it works perfectly and even quite snooth when running backwards, I'm going to try mounting the differentials reversed, so the whole drivetrain runs reversed.
EDIT2: I've reversed the diffs. While doing it I found one of the small blue pieces underneath the car, behind the front axle was a bit loose and tightened it. After reassembly it works on all gears. Surprisingly, it doesn't crack if I move it backwards (the previous forward) on 4 Hi. In any case, I think mounting the diffs reversed is worth it as the transmission is A LOT smoother and has way less drag when running reversed. Moreover, its really easy to do with almost no disassembly involved.