The purring sound of a cat is produced by the vibration of the false vocal cords. Generally, cats will make this sound when they are relaxed, comfortable and happy. This is a normal phenomenon and there is no need to worry. Cats' purring also has the effect of self-comfort and healing. Cats will also make this sound when they are uncomfortable, in pain, or nervous. Owners should pay attention to observation and seek medical attention in time if the cat's body is abnormal.