If the fans are connected to the MB, then you should have options for speed in the bios.
You could try to isolate it by disconnecting all but the cpu fan and listening for the sound.
My sound was not too bad, but I did have a fan that did it louder and over a larger range of speed. 1 RMA later and this one just has it in one spot(within maybe 40-50 rpm) and most users would not hear it.
It was for me more when the system was idle and cool it would sort of oscillate its frequency(with the slight change in speed) and that was the part of it that made it noticeable.
I used my MB bios to set a slightly higher minimum fan speed(3% more speed at idle).
ngengerous, I had an OCZ do that well after warranty. I swapped that sucker out. Problems solved