Snakes make a hissing sound as a way to defend themselves. This is especially important for smaller, non-venomous snakes, which use hissing to create the impression of being larger and more dangerous. Other defensive maneuvers may include biting, emitting musk or retreating.
Snakes breathe through an organ in the throat called the glottis. With normal breathing, which usually occurs quietly, the glottis opens and closes to control the flow of oxygen. When a snake is threatened, it forcibly expels air from the glottis, which makes parts inside the glottis rattle to produce a hissing sound.
Although all snakes are capable of hissing, many species are not known for using hissing for defensive purposes.