Question Whistling Noctua fans - new case required?

[Libita]

I corrected some errors recently that had my Noctua NF-A14 fans running at about 20% in my Fractal Design Define R5 case. Since then, they sometimes produce a whistling sound depending on their speed.
I tried to

• open the front door,
• remove the intake filters,
• manually closing some holes in the back outtake and
• getting some spacers so there's more room between the fans and the filters and/or holes.

To no avail. Something is always whistling.

So, now I am thinking I could try to get another case if that would help. But I have no idea what to look out for, so that I don't get that whistling sound again.

Does anyone have any ideas on what to look out for? Or is this the wrong solution?

 

[Lafong]

Is there any correlation between whistling and RPM?

I have some A12 Noctuas and don't have that issue.

I'm guessing they whistle only when installed in the case and not when simply connected to power and spinning outside the case.

Is that true? If it is, I'd assume it is a peculiarity of either your case or the frequency response of your ears.

I suppose it could also be an aberration or defect on your specific fans....manufacturing issue.

 

[Libita]

The sound is at least somewhat dependent on RPM, since it doesn't happen on low RPM. It also only happens when they are installed, not when I simply put them on power outside the case, so I am pretty sure it's an interaction with the filters and/or holes in front of the fans.