It is not the rotors, as they do not touch anything, not each other, not the case, nadda. There may be some gear noise, but the bulk of the sound is from the supercharger compressing air. The screw type supercharger is the only one that actually compresses the air as it travel forward through the screws. The roots (Magnuson, TVS, Eaton et al) blowers do not compress air internally. They just move more air from top to bottom so to speak. Same with centrifugal blowers and turbo's. They just push air from one point to another. Some of it is the inlet and outlet manifold material, shape and thickness. Much like a Ferrari's sweet sounding exhaust has more to do with their use of titanium manifolds/headers than with the actual engine. Although high rev's certainly helps, and of course a overhead cam engine sounds different than a pushrod engine ect.
Side note: When we did get the LC from Kenne Bell, they sent the wrong one! We had ordered the "H" and they sent the "S". Had to send it back and get them to send the right one. They even paid the shipping costs!