Hmmmm. The Nocuta (or any other brand or DIY) fan resistors are intended just to reduce the speed (or rather, scale the speed range) of voltage-controlled fans, and reduce the sped of PWM fans.
But I wonder if sticking a resistor in there has another effect. The power controller on the motherboard dedicated to fan control is probably not all that sophisticated, and neither is the BLDC controller in most fans themselves. If either of these are shooting transient signals at each other, those could end up bouncing about between the two. If those transients are resonating in the audible range, you could get weird 'fan noise' that is actually modulation of the fan motor itself rather than noise from the fans normal operation. Sticking a resistor in there is going to change the time-constant of that system, which could move any transients out of (or into) the audible range.
Man, that would be a pain to test though. You couldn't just put a fan on a bench supply, you'd need a bunch of motherboards to test with just to find if there is an issue in the first place. And because the resistor is also reducing fan speed, you couldn't rule out that the fan was perceptually sounding better just because it was running slower. Maybe the best way to test would be for someone who is already getting annoying fan noise, and stick a smoothing capacitor across the 12V & GND lines. That would have a similar effect as a resistor without changing fan speed.
::EDIT:: Or maybe I've spent so much time hunting bizarre gremlins in systems that I see them lurking in normal responses.
