NH-D15 weird noise

[Rainbrew]

Hey, so I've been running an NH-D15 in my PC for a while now and as my be-quiet case fans were starting to get loud I decided to swap them for some 140mm noctua case fans. Work great, whisper quiet, super happy. However I noticed that under any CPU fan speed above ~30-40% my NH-D15 would make a very weird noise. I made sure that the cooler is mounted properly, that the fans are properly attached, have a custom fan curve for minimal noise, and made sure that I didn't over-tighten the case fans. I also took the fans out of the cooler and tested them at varying speeds and neither makes abnormal amounts of noise or vibrates significantly. Offsetting the speeds of the two fans also did not change the hum.

As far as I can tell the turbulent air somehow induces some sort of resonance into the fins causing a very odd pulsating hum. This would be a non-issue as the PC is quieter overall however the noise is high pitched enough to cut through my headphones in 90% of situations and is extremely annoying. Attached below is an MP3 of me running my PC for a bit and turning on cinebench. My budget mic had some issues picking the sound up so I had to turn it up quite a bit, but the noise is audible throughout most of the clip especially at the ~39s mark. Any clue what causes this or how to mitigate this?

 

Also VOLUME WARNING and thanks in advance. : )

[Origami Cactus]

Could you just limit the cpu fan speed to 40%, it is an insanely good cooler so shouldn't be an issue.

[WereCat]

The high pitched sci-fi pulse?

 

Souns like when you try to blow air into a bottle from a side... definitely annoying and I have no idea what it could cause it unfortunately. 

 

If fans alone don't make this sound then the cooler can't be an issue though.

Does this happen when the side panel is removed? Maybe the fans are forcing the air trough some gap there making the sound?

[Rainbrew]

24 minutes ago, Origami Cactus said:

Could you just limit the cpu fan speed to 40%, it is an insanely good cooler so shouldn't be an issue.

 

14 minutes ago, WereCat said:

The high pitched sci-fi pulse?

 

Souns like when you try to blow air into a bottle from a side... definitely annoying and I have no idea what it could cause it unfortunately. 

 

If fans alone don't make this sound then the cooler can't be an issue though.

Does this happen when the side panel is removed? Maybe the fans are forcing the air trough some gap there making the sound?

I FIGURED IT OUT. I played with the fan settings in my bios til I heard the noise and I discovered that at certain speeds the air flow induces resonance in the top of the case (nowhere else). The rest is fairly bulky in the be quiet 500dx but the top part is mounted with a few rivets and is a fairly thin piece of metal with a triangular grille. I don't have a top exhaust fan so I'll see if I can somehow cover those vents and if it'll stop making the noise.

[Rainbrew]

So as an updated I re-positioned one of the fans of the NH-D15 a bit lower and added low noise adapters to both and while it is quieter the noise still persists. Can't do anything to the case to make it stop doing that so I'll contact noctua and be quiet and figure out if the case is funky or maybe one of the fans is vibrating abnormally.

I noticed that the case fans even at 100% experience no vibrations while the two CPU fans do vibrate at 50%+ but that may be due to the higher rpm so idk.