Irregular fenestron blade spacing
The reason that the fenestron has irregular blade spacing is to do with the blade-wake interaction - but to make it irregular, rather than to reduce it.
To minimize the strength of the interaction, the blades should be as separated as possible.
The irregular spacing cuts down on the high-pitched whine that's very pronounced from a Gazelle - the equivalent of the typical buzz-saw noise from an old large turbofan at high power. The new fenestron produces no constructive interference in harmonics at any multiples of the fenestron fan rpm frequency, and so makes the helicopter sound quieter, by giving the fenestron a quality of tone that is richer and less penetrating and annoying.
I believe this is the same reason that the Apache has a 4-bladed tail rotor in a squashed rather than right-angled cross configuration.