Does the noise coincide with his breathing? You can observe the breathing like you would other animals -- watch for the body wall movements with inspiration and expiration. If the noise does coincide with regular breathing, I would get your friend to a vet.
Hissing is accomplished by forcing air out rapidly. It is a purposeful noise, and I believe it is pretty obvious what it going on to the observer. In my experience, snakes typically hiss at the onset of handling, and the hiss often is accompanied by balling up, reeling back into strike position, or other defensive behaviors.
Something else that can cause respiratory noises is a stuck shed on the nostrils.
Another consideration is that the snake is stressed by handling, stress causing an increased respiratory effort.
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