I recently had a pair of NF-F12 PWM fans delivered for use in push/pull on my Corsair H80 radiator. I was previously using a pair of Akasa Apache's.
I was initially disappointed as when running at full speed they were much noisier than the Apaches and produced an annoying whining sound, whereas...
I had two Apaches in push/pull on the H80. I needed a couple of case fans so I decided to replace the Akasa Apaches with the higher speed Akasa Vipers and move the Apaches to be case fans.
Long story short, the Vipers are a little bit too noisy for my liking and I will be returning them...