When learning the intricacies of the Victorious Breath (Ujjayi Pranayama), students often find it difficult to produce the correct sound characteristic of ujjayi. To do this, air is drawn in and out through the nose, but the sound should not come from the nostrils. If it does, you are in effect sniffing. When you are moving to the rhythm of the breath, your muscles demand a constant supply of oxygen. To meet this demand, airflow needs to increase - but if you sniff, airflow is, in fact, restricted. To prevent this, each breath is drawn in from the back of the throat so that the airflow can be increased and metered by the muscles around the glottis. It is the friction of air through the glottis that produces the ujjayi sound. This friction also warms the air before it enters the lungs. To help correct any tendency to sniff, lightly stretch the skin on either side of the nose to dilate the nostrils so that the air is being drawn in at the back of the throat.
The correct ujjayi sound is simlar to the noise waves make as they surge up a pebbly beach. To achieve this "free breathing" you must keep the glottis open at all times during the inhalation/exhalation cycle. Closing your glottis is like holding your breath; if this happens, the energy flow stops and your muscles become starved of oxygen and pranic energy, and therefore tighten up. In this state, it could be said that "where there is no breath there is no life," and thus vinyasa and the asana become lifeless.
The grunting noises practitioners sometimes make indicate that the glottis has been locked closed - this usually occurs at the top of the inhalation and the bottom of the exhalation - and so you need to refocus your attention on keeping the glottis open.
-John Scott in his 2000 book Ashtanga Yoga