The centri chargers are almost quiet at WOT, but tend to be noisy at idle and cruising. They now have quiet versions , but they still have a definite blower whine at idle speeds.
As far as boost RPM, with those, boost is directly dependant on RPM. Its a straight line from 0 lbs at idle to the max boost at redline (give or take a hair of PSI). At 3k, you may have 3#s, at 5k you may have 6#'s, and at 6500, you may have 8 (just for example). Most of the new curved impeller designs can compensate and tend to make a little more down low, but none can come close to making the low end grunt of a Positive displacement supercharger.
