A Rattlesnake's most distinctive feature is its rattle.  A long-standing myth is that a rattlesnake adds one segment for every year of its life.  This is simply inaccurate.  One rattle segment is added every time the snake sheds, commonly three or four times per year or more.  Another myth is that the rattle itself is dangerous.  This is not true either, it is made of the same material as the skin and carries no toxins of any kind.  It is true that a snake does not need to rattle in order to strike, but usually the snake does rattle as soon as disturbed.  Rattlesnakes often lie on their rattles or get them stuck in crevaces and as such the string breaks easily.  To me the sound produced sounds less like a rattle and more like a constant buzz.  More than one snakehunter has mistook the sound of a disturbed Timber Rattlesnake for the buzz of cicadas.
    Rattlesnakes rattle not because they are angered, but because they are frightened.  A heavy footstep or the approach of a predator (includes humans) sometimes is enough, but usually they must be disturbed before making any kind of sound.  Other harmless species also shake their tail when frightened.  These include Milksnakes, Rat Snakes, and Racers.  The object here is not so much to imitate the Rattlesnake as it is to startle the predator and draw attention away from the head.  Not every tail-shaking snake is a Rattler.