I saw something on the rattlers in an entire county not rattling. There was an old guy that hunts rattlesnakes as often as he can, usually by sound in probable locations. The theory is his hunting pressure has left only the quieter snakes to pass on their genes so, over the last few decades snake in that area have become quiet rattlesnakes.
His fear of what he doesn't understand is in effect endangering more people as the snakes do not try to warn you off but, instead are more likely to freeze then strike if you seem to be posing a threat.