Radiator fans making oscillating noise

[Netherspark]

There's this very quiet but inescapable oscillating 'hum' coming from my radiator fans and it's driving me crazy. It's this slight humming sound that alternates constantly between two different tones, twice per second.

I stopped the top fan just to test it and the sound went away, so the logical conclusion is that the bottom fan is the culprit. But then I tried stopping only the bottom fan and the noise also went away.... The sound only happens when *both* the fans are spinning.

The fans are brand new btw - NZXT Aer P 140mm.

Can anyone help explain why they are doing this, and better yet tell me how to stop it?

 

[13thmonkey]

it sounds like a beat frequency between the two. you need to change the speeds slightly between them (if possible), or interrupt the sounds waves so that they don't meet. Test a small piece of card sticking outwards between the two fans like a baffle. If that works a more elegant solution is needed.

 

[Netherspark]

Yeah, putting something between them does stop the noise. So I guess conflicting frequencies is the cause.

The fans are controlled by the AIO and run from the same cable, so there's no option to alter their speeds individually. I suppose I could try adding a low noise adaptor onto one of them. I'd prefer a better solution though.

 

[Sedivy]

Hmm this might be risky but if you have spare mobo fan headers, you could plug them directly into them, in bios set the fan header to respond to cpu temps, and then just control their speed via whatever utility comes with the mobo. You'd have to set this all first up while in bios, while keeping an eye on temps, to make sure whatever curves you set cool the cpu properly for the whole range of temps. Then adjust from within windows for higher temp ranges. That way you can give them individual curves.
I don't know if pump needs fans to be plugged in directly to function and if the nzxt's utility would freak out once fans are unplugged from it, so that might be a thing to ask nzxt.
Just a thought...