basically, i am not know much about bikes engine. i got a question when i riding bikes in downhill with lower gear the engine sounds very noisy I think engine runs more than the ideal speed, my question is it consume more fuel as it inlet and outlet valve opens very quickly, if there no supply of fuel why does engine sounds or how does engine runs more than ideal
Riding in gear with the throttle off is the moment you'll burn the less gas ever. It's less consuming than staying in neutral and letting the bike going by itself.
The loud noise is just the engine being revved by the rear wheel and that is perfectly fine. You can downshift if you need more engine brake, getting lower speed and ultimately lower rpm if engine brake slowed you enough.
EDIT to support the fuel theory : https://www.popularmechanics.com/cars/hybrid-electric/a5977/coasting-in-neutral-fuel-economy/