Your question continues to elicit a surprising amount of argument among reasearchers. Most researchers call these muscles that line the larynx vestibular folds, or false vocal chords.
Air passes over the vocal folds with the inspiration and expiration of the diaphragm.
It's rather like snoring, say some experts. Dr. Paul Orsini, who teaches anatomy and surgery at the University of Pennsylvania School of Veterinary Medicine, puts it a bit more elegantly. He compares the mechanism of the purr to how sound is created by the air burst blown through the reed of a clarinet or other wind instrument. Due to the large volume of mail we receive only questions for publication can be answered. The official Web site of Popular Science Magazine.