A growling cat is an unhappy cat. This low throaty rumbling can occur when a cat is warning off another cat, or if its pet-parent has gone way too far. Cats in this state-of-mind will show other signs of anger like raised hair, flattened ears, wide eyes and a twitching tail. If your cat is in this stance, just leave him alone, unless it is in danger, then do what you can to diffuse the situation.
9 Sounds Cats Make and What They Mean