Got my R909 unit yesterday.
It was equipped with an Artic F8 fan, that has a good airflow : 52.7 m³/h at 22.5 dB
Replaced by a Noctua NF-A8 PWM : 55.5 m³/h at 17.7 dB
Are you sure that you got the PWM fan? If so, it is now running at 100% fixed fan speed, since the R909 only outputs a 3-pin DC fan signal and Noctua PWM fans default to 100% fan speed if they get a (any) 3-pin DC fan signal.
@sidehack does the R909 have any fan speed control at all or is it outputting a fixed 12V anyway? In that case, it wouldn't matter (at least in case of Noctua fans!) which type we choose.
I need to replace my fan, too, since I find it too loud (at least the
type of noise it's making is annoying me, somehow).