A couple of earlier contributors likened the powerplant noise to that of a flight of Harvards.
The Harvard racket comes from the tips of the two-bladed props going supersonic.
A change to three blades fixes the problem - probably a major reason for the change to three-bladers on the FXXXVI.
p.s. DHC Beavers operating in noise-sensitive locations make the change from 2 to 3 for that reason as well.