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.
Chemically some metals are more reactive than others. This can be deduced using the reactivity series of metals, which is an arrangement of metals (and carbon and hydrogen) from the most reactive to the least reactive. Any metal will displace another metal lower on the reactivity series from a solution of its ions. This does not apply to any metals that react with water, so can only be applied to magnesium and below. Using the displacement theory above we can also include carbon and hydrogen in the reactivity series.