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.
Memory's fading a bit here but I think the Vanguard T/O power LP RPMs were 15050-15500 and the max cont HP RPM 17,900 (LP 13500). Prop RPM <1000 at T/O power (950 rings a very faint bell).