Got 6 new shiny AP-14s. Tested and mounted 4 so far on my new Corsair H100i. Here is what I have learned while testing the fans: they start producing a very noticeable tonal sound when I block airflow from the intake side by, say, putting my hand closely over them.
Now, the two intake push fans over the radiator are doing fine--I have Silverstone fan filters/grills over them, but those don't impede airflow enough to create this sound. The two pull fans are pulling the air from the radiator and are rather close to it. I've added some rubber grommets for vibration dampening, so along with those and the frame of the radiator, there's maybe a third to a half of a centimeter of space between the fans and the actual radiator fins. Those two pull fans make a noticeable tonal noise.
Is there anything I need to do to help it go away? I'm going to connect all the fans to my fan controller tonight (they are working through Corsair's H100i pump right now, and it's not doing a good job of keeping them at the same RPM and throttling them properly, since they are not PWM fans) and play with various speeds, to see if I can get the sound to go away, but I'm also wondering if I should insert fan shrouds or some sort of extra spacers between the radiator and the pull fans. I figured this is a common issue for these fans, since most people buy them to go onto radiators, so what is a general recommendation for silencing the annoying tonal noise?
Update: connected to my fan controller, the resonance is much less noticeable, and there's significantly less of the tonal noise. Now that I could control each fan individually, it comes from the top intake fans as well. Have to run them at low-ish RPM anyway, or they suffocate my GPU (small case).
Edited by ElevenEleven - 11/28/12 at 4:44pm