While Jerome is correct on the air getting trapped in the belt...there is quite a bit of noise from the head units themselves. What you here there is the gear noise inside the actual supercharger.
If you order just the head unit from a supercharger company, (I.E. ProCharger, Paxton, Vortech, etc.) you can specify whether you want one on the quieter end of the spectrum, or the louder end. They test them when they are built. A lot of f-body and mustang guys do this for different purposes. Quieter for a sleeper approach, louder for the "I am Lion, here me roar!" approach.
If you order just the head unit from a supercharger company, (I.E. ProCharger, Paxton, Vortech, etc.) you can specify whether you want one on the quieter end of the spectrum, or the louder end. They test them when they are built. A lot of f-body and mustang guys do this for different purposes. Quieter for a sleeper approach, louder for the "I am Lion, here me roar!" approach.