The fans are regulated to maintain positive pressure inside the head. This is to prevent fog fluid, dust, pyro remains from being sucked into the projector. When you open the fixture up you will actually hear the fans get louder as the ramp up to maintain positive pressure. The quietest operation will come from making sure the filters are clean and everything is in place and sealed correctly.
There are plans for simplifying the fan operation in the next release by removing the requirement for calibrating the filters when they are changed. This may or may not have an effect on the noise level of a specific fixture as it depends where the calibration values were before.
