I’ve spent good amount of time over last few days trying to identify what is causing annoying sound from system I’ve been building that, in the lack of better description / sound file, I cold only describe as “second wave form” (in addition to regular fan noise) that would intermittently come on, start rising, then start falling down until it would go away and later repeat itself. Kinda like an intermittent prolonged lazy low volume “howling”.
 
I couldn’t easily identify a source but method of elimination pointed at middle fan on CLC360 as suspect.
 
To check is it really middle fan I’ve changed EVGA Flow Control from default auto fan curve to fixed RPMs and cranked up RPMs to max. That made “second wave noise” go away. Which made me think middle fan might be having problem maintaining lower RPMs.
 
To check that I’ve manually lowered fan RPMs to exact same level they were running at when I would get the noise and now I wasn’t getting it. But moment I switched back to auto fan curve noise came back!
 
That made me go to curve editor and change it from “curve” to practically a straight line. Noise went away.
 
At this point only things I could see resulting in that kind of noise from middle fan are:
 
1) Middle fan is “malfunctioning” and is not able to maintain speed, or is at least not responding smoothly to issued levels / not “working in harmony” with rest, or
 
2) Fan curve is somehow triggering it, or
 
3) Combination of both
 
So does anyone have similar experience or has some input they could share, please?

AngryAce
I only have a 120 AIO for a ITX build. On that motherboard, if I don't manually set the cpu fan mode to PWM, the fan would do all kinds of weird stuff from not working at all, surging or 100% RPM. Shot in the dark  since you have a 360 but you can check that and see it does anything

I don't know which mb you are using but for my X299 Dark instructions call it should be connected to CPU1 or CPU2 and that one should be set to MAX which is PWM. So I don't think that would be the culprit.

Noctuas have arrived and are in (while at it I’ve used the opportunity to replace fan on my 2080 Super Hybrid too).
 
So far “2^nd wave form” is (to knock on the wood) gone but I haven’t had them on long enough to be able to say with certainty. If that continues to be the case then culprit was “just” one of the fans and fan curve changes were “hiding” it.
 
For those that might be curious how Noctuas are performing: I’ve run smallest FFT Prime95 and  TimeSpy Extreme as stress tests and my impression so far is that using them buys you more time before highest temperature is reached but once it is reached there is no difference in CPU/coolant temperatures between EVGA and Noctua fans. “Only” difference is Noctua’s are running at lower RPMs / quieter.
 
Which makes sense when you remember laws of thermodynamics. No other variable involved in heat transfer has changed (still same environment temperature, radiator, liquid, pump …) so as long as both fans are good enough to transfer that (and obviously they are) you will get same heat transfer. Rest (how long it will take you to get there and what will be noise levels along the way) will (assuming same fan curve) depend on how efficient one fan is versus the other at which rpm / noise level.

Hoggle
The new fans should help fix the issue since they will run at a lower RPM and be quieter. Would be interesting to see which fan has the higher CFM rating but Noctuas fans are a great choice.

I might be wrong, not having enough experience and knowledge in this area, but my gut feeling is that higher flow rating matters only when you are pushing your cooling to the limit (blast fans to the max and noise be darned) and in all other cases adjusting fan rpm curves will make fan A push same volume of air as fan B at same coolant temperature so “only” difference one will see is in RPMs at which they will spin at that temperature (and consequently noise levels).
 
In my case I am not trying to squeeze every last drop of performance out of the system and quietness matters a lot so I am more curious about flow vs. noise level curve across its whole range.
 
Thus I didn’t even bother trying to find which one of these would let me push system harder, for me not getting worse temperatures while getting more pleasing sound was all that was needed to make me happy.