Did some more details on the Truck. First, I hided the wiring in the cabin, so this looks much better now:
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Then I put a Poison Truck motor in and re-installed the gearbox. The guy who build this one first overturned the schrew in the pinion-gear that bad, that I had to drill a new hole and cut new windings!
But even after overhauling the gears, they still "sing" very loud. I guess, the only solution will be in changing all the gears some time!