All the 331s howl like a banshee don't they!
The MU-2 does so most distinctively indeed. The inlet on the rice rocket has the shortest distance from the inlet lip to the compressor of any of the 331 installations. This may explain the difference in sound from Commanders, Metros and Jetstreams. Those funny harmonics vanish the moment the speed levers are pushed forward into the takeoff range though. And the noise output diminishes significantly as the torque is applied for takeoff. To my ear, the 331s make less noise at takeoff power than the PT6. Too bad about about the ear-splitting noise at ground idle!