Cats, although usually cute and sweet, are also very mysterious and confusing creatures. If you are "owned" by a cat yourself, you've probably spent many a sleepless night scratching your head trying to decode your pet's puzzling actions. Growling and hissing behaviors are certainly no exception.
Although growling and hissing usually come across like angry sounds, they are often actually indications of fear and vulnerability. For instance, your cat may growl or hiss in the presence of a new, unfamiliar and bigger cat. Perhaps he simply feels like he needs to defend himself against a possible predator attack -- understandable!
Hissing and growling also can point to pure annoyance. You may think that all is fine in the world and that you're just enjoying a cozy night holding your cat on your lap as you watch television. But if you suddenly notice that your cat is squirmy and growling or hissing, he might just be communicating to you that he's simply had enough contact and wants off your lap -- nothing personal, of course!
Cats also often hiss when they are caught off-guard by a predator or anyone they perceive to be a threat. For example, if a new and strange cat shows up on a cat's designated "turf," he may instantly begin to hiss as a defensive and territorial instinct.
If a cat lets out a gravely, deep and prolonged growl, he probably is attempting to send out a serious warning. He is either angrily communicating to others to back away from him, or perhaps even to move in another direction away from other approaching danger -- a German shepherd, for instance. The growl often means that if you don't respond to his signal, he might just take out the claws or teeth -- yikes!