It is a popping resonating sound generated by a lean condition on deceleration. When you suddenly close the throttle, the incoming air is choked off and the closed throttle also causes the bike to return to idle on the very lean idle circuit.
The reactivity series of metals was deduced by performing several experiments in the lab which enabled scientists to arrange metals according to their reactivity with dilute acid, oxygen (air), and water.xides of iron and copper are insoluble. Metals that are less reactive than copper like silver, gold and platinum do not react with oxygen. Note: When aluminum reacts with oxygen, a layer of aluminum oxide adheres and covers the aluminum.
Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, domino fashion through the conductor. Simply stated, most metals are good electrical conductors, most nonmetals are not.