superdan54
I'm trying to get my computer whisper quiet, but still have one obstacle to overcome. I have a single Noctua NF-12 120mm fan and a Corsair H60 liquid CPU cooler, all housed in a Fractal R4 silent case. This has helped cut down noise considerably, however I'm still hearing the fan motor noise pretty significantly as it's amplified by the chassis. It's at a specific frequency, so I'm worried that introducing that extra noise will cause me to artificially cut my mixes there. However, I have no idea how to eliminate it. The fan motor noise is definitely coming from the Noctua, as unplugging it makes the pc virtually silent. Also, it's brand new as I just got it to replace the stock Corsair fan that came with the cooler. I was also getting the motor noise with the Corsair fan. Finally, I've tried adding some rubber washers to the mounting screws but that didn't seem to help it at all. Is there anything else I can do? It's one of those deals that by itself, it isn't very loud, but just noticing it makes it extremely loud in my mind and drives me crazy!

If the Noctua fan is audible when the case is closed, I'd replace it.
 
Are you sure the resonance you're hearing isn't HD related?
Sometimes, HDs (even when mounted on rubber grommets) can cause significant case resonance... which results in case vibration noise.  Moving the HD/s to a different slot can help de-emphasize this resonance... and prevent the noise.

Multiple fans running at lower speed typically make less noise than a single fan at high speed.

Noctua fans are usually very quiet indeed, especially if run on reduced voltage. I've had several over the years and out of the many "quiet" fans I've tried they're about the quietest in the same way the big Noctua coolers seem to be the most efficient. I usually aim to run them at around 5V, using a controller if necessary to up the voltage at boot/wake if ghe fans won't start at 5V (some won't). I also pick the biggest fans possible. The PC in my sig has two fans and never overheats and is almost silent (about 24dB at 1 metre). One fan is on the cpu cooler but working "backwards" to the way Noctua's instructions say and the other is an extraction fan at the back of the case.

I don't use automated fan speed control at all, they just gently tick over all the time. But when I build a PC DAW I do quite a lot of experimenting with fan voltages/speeds and placement while temperature monitoring as part of the process until I find what seems to be the optimum for that particular build. Once I can hold the cpu at 100% for ten minutes without getting anywhere near the cpu's thermal rating or into cpu self-throttling I generally reckon it's getting enough air.

What can sometimes happen is that if there's two fans, e.g. cpu and a case fan, the two can set up beats (like two synth vco's slightly detuned) between each other that are louder than the combined noise when they're not beating. I've found that moving the case fan a little or making a small voltage adjustment to "tune" the motor speeds can make a huge difference to this.

Case resonance can also be a surprisingly loud noise source. The "sound proofing" kits sold for cases can help with that, though they won't prevent noise escaping despite what the manufacturers say because noise will get out of any hole. They can be very useful for damping vibrating panels though. An alternative that might be cheaper is the self-adhesive matting that's used for sticking to car floor-pans under the carpets to dampen panel resonances.

Playing around with the airflow in the case can also make a big difference to noise levels. My previous PC ended up with the case fan blowing air in at the bottom rear and exiting through the top 5.25" front drive bay. An odd setup but it ran much cooler than a "conventional" approach which meant I could reduce the fan voltages and hence noise.

Since more 2 years now I use to put Be quiet.
A little bit less expensive than Noctua.
 
I own an Fractal define R5.
 
With this configuration I have very quiet PC.