The purr is probably the most singular sound in the feline vocabulary. Unlike the meow, which some people pride themselves on mimicking reasonably well, the purr is a sound that can't be duplicated with the human mouth and vocal cords. The whirring sound your cat makes when he purrs is created by air passing over his larynx both as he inhales and as he exhales. It's an effortless sound to make; he could carry it on indefinitely if he wanted to. The purr is one of the most misunderstood feline communications. Many cat owners who thought their cat was expressing contentment have been shocked to get scratched or bitten in their confusion. Cats do purr when they are content. When a mother cat is purring, her kittens are comforted by it. When the kittens purr, she knows they are content and nursing.
Cats can also purr when they are in pain or discomfort. That discomfort can be caused by a physical problem (purring in labor is not uncommon), or it can be caused by an uncomfortable situation. For example, when your petting hand has worn out its welcome, the cat will purr louder and more intensely. The cat will also put its ears back and fan out its whiskers so they come forward. This means the cat is saying he's irritated, not "I'm enjoying this," by a long shot. "We don't know which emotion is affiliated with an aggravated purr, but it seems to result from over stimulation," explains Dr. Wright. "The position of the ears and whiskers is the best cue owners can watch to avoid being scratched or bitten."