Question about Arctic P12 Value Pack

Yes, the P12 is a good case fan, I use 4 in my pc (3 P12 + 1 P14). Anyway the whole airflow and airpressure discussion is dodgy. The behaviour is nice, exactly the same as a PWM fan but cheaper (except a PWM fan doesn't need to ramp up at boot up). The fans are mostly very silent for their speed except I get some vey low whinning, speed related (it could be my motherboard). I did change one of my fans by a spare one but today I noticed another noise which reduced by changing speed (I'm a nerd).