I suspect that the very high RPM of the Dart was a major contributor to the noise - Dart 510 ran at 14,000rpm while the 520 ran at 14500 at max power. Centrifugal compressors require to run at much higher rpm than axial. The Garrett TPE 331 in the Skyvan runs at a max of 43,000rpm - I saw one let go on a max power run it was spectacular to say the least.