According to Dr. Sherlyn Spooner, Montreal Veterinarian, there are two theories as to how cats purr. The older, less-accepted theory is that vibrations of the blood vessels of the cat's throat cause the purring noises. Now, the more accepted theory is that the purring is a result of aerodynamics. During purring, there are small muscles attached to the cat's voice box that contract.
Air is pushed rapidly out through the glottis as the cat breathes out.
That's what makes the purring sound. Well, nobody really knows. Purring is a very voluntary action - cats choose when to purr and when not to purr. They usually purr when they are happy - when you rub their tummies or when they get fed. You can hear some examples of these cat sounds in our interview with Dr. Spooner.