Having got a whole set of Arctic P12s (what most people favour for price to noise and performance) in one rig, and a whole set of Noctua A12x25s (formerly with A14, same as your one, except PWM and non Chromax, and an A20 and a A9, all PWM) and formerly other fans as well, I can drop a few things of note here:
I can confirm that I too ran into a few issues with the A14 in one of my cases, initially it turns out it's not the fan, but a PWM issue on the first A14 fan, then the replacement when used with the dust filter (soft version, that's held there by duct tape) and the cage I had attached the A14 to, was not actually stabalised on the z axis. This resulted in the fan creating a sort of hum, or harmonic resonance type of sound from it when dialled up. So I agree with
@C64 that it may be possible that something else is causing the problem that you're not aware of. Once I had that sorted out (through various means), I didn't have the same issues crop up on the cage and its z axis vibration resonance, but did find that the A14 at higher rpms caused a rush of air sound that was not "smooth" when fitted inside the case (it was fine outside the case), this is what you are experiencing I imagine. And in that case, I agree that the A14 isn't very nice.
As above though, you should just do some quick checks to make sure there's not other mechanical issues that you might have missed that is causing this (like the z axis on something isn't acting up at the front of your case, etc). But even here, I swapped the A14 out for a Noctua A12x25 for a smoother and lower sound output when I need to push the rpms (I don't really, but just in case). But in your case, you're probably best served by going for 140mm fans, which Noctua do not have a product for that is a contemporary of the A12x25 in the 140mm band (it's been pushed back a year, again, supposedly named A14x25), I'd recommend you try the Arctic P14 as someone else suggested earlier for the 140mm fan slots at the front of your case.
I personally have not had issues otherwise with the Noctua A12x25s, other than it's silly pricey. And if I wasn't curious as to how low I can get the noise produced from this rig going air only, I wouldn't have gone the whole nine yards for it, and indeed also would have just got more Arctic P12s. But, my recent tests also showed that there's some discrepency (in favour towards the Noctua) between the two fans at various audible rpms (The A12x25s just nudges the P12 where it needs it to). So I would say the Noctua fans are good fans and (personally) the best available (but I can only say this for the A12x25).
