Sound consists of wave motion in an elastic medium and caused by the vibrations of molecules. It is periodical mechanical disturbance in fluids and solids. Noise radiates from vibrating surfaces as well as from vibrating gases. Moreover, noise also grows rapidly with an increase in the speed of exhaust gases.
In other words, sound is produced in environment by alternating pressure changes in the air and is caused by the vibrations of solid objects or separation of fluids, as they pass over, around or through holes in solid objects.
These vibrations cause the surrounding air to undergo compression then rarefaction, and so on. Such alternating compression and rarefaction of the surrounding air produces sound waves which propagate in the form of sinusoidal path.