They sound this way because they straight pipe only one of the cylinders, directly from the manifold and the rest goes the normal exhaust way. It's called maniwari. Just learned about it and it's facinating.
I think thats because the true-dual exhaust setup which create rumbling harley-like sound, Indonesian truck driver do that too.. They called it "stereo pipe" Setup
1:48 Parece moto xD saludos desde Paraguay alguna pagina web para comprar asi estos camiones mi sueño es tener estos camiones yo jugaba mucho en la play 1 un juego de simulador del dekotora