Howling AIO fans, what could be the cause?

[robgmun]

Hi all!

So I just finished my build, all seems to be good except for one small thing. My case is a Fractal North and the AIO is an EK Nucleus 360. I put the 3 fans in the front of the case ahead of the radiator pulling air into the case. I've noticed a strange sound from a distance. It's best described as an intermittent, very faint distant howling wind, barely noticeable and when I have anything playing on my PC like a video or music you don't notice it. But when the room Is quiet you can faintly hear it. It's intermittent and comes and goes every few moments.

It is annoying and I want to resolve it as there are periods of quiet in my room as I do work. I've traced the sound to the front fans because when I stop the fans with my fingers the sound stops instantly. If i only stop one i can still hear it, it's only when I stop all 3 does it actually stop.

Can anyone shed some light on this? The system is completely new and very clean so there's no dust anywhere

 

[robgmun]

There's various forms of resonance that you need to combat as well if it is resonance you're experiencing. One such version is harmonic resonance, caused by fans that all run at the same RPM or close enough to each other, so that after a certain amount of time, resonate at the same moment and cause the noise. This form of resonance can be combated by individually controlling each fand and running them at slightly different RPM from each other, which on an AIO might not be as easy to do, as usually they all run at the same RPM.

So I kind of resolved it. At the time of the first post, the 3 fans were running at 20%, so I gradually reduced the fan speed until the noise disappeared, which turned out to be 15%, so I've set it to run at 15% unless the CPU runs hotter than 40c which means that if I do anything that stresses the CPU will most likely involve noise like playing a game or watching a video. Problem solved I guess!