Impulses from power and compression strokes cause torsional vibration and there is also a basic problem with 4 cylinder motors except V4s is as unlike 3,5, 5 and 8's is all the pistons are accellerating and decellerating at the same time, causing the opposite reaction on the crankshaft. At low speeds and especially with no load this causes teeth in the gearbox to chatter back and forth. The more free play/ backlash in everything, the more damage it does and the faster the problem gets worse. Includes wear on the clutch plate and front gearbox shaft splines and maybe even a bit from slop in big end and gudgeon bearings in a worn motor.

Maybe the reported problems of Isuzus in Landys destroying gearboxes at low speeds would not be as bad if the flywheel was heavier. I have not listened closely to one in different places but would imagine at slow idle part of the overall knocking noises come out of the gearbox.

This reminds me of a problem with the riddle box/cleaning shoe/ seives on combine harvesters - generally known as headers in Australia. It shakes back and forth at about 280 cycles/minute. If any backlash occurs from wear or failure of a bearing in the crank setup or seive coming loose, the resultant hammering rapidly cracks things up.