I recently replaced my Antec Solo case fan with a 120mm, 800RPM Scythe S-Flex, which I run at a constant 12V / 800RPM. Overall the new fan is quieter, as expected (it is rated at only 8.7dbA), however the noise is no longer a quiet whoosh of air. It has taken on a distinct pitch of about 1kHz, right in the sweet spot of human hearing, particularly when starting the machine from cold. Is this normal for this brand of fan or did I get a lemon?
 
The fan is connected to the CPU fan header. I have a passive CPU cooler with a fan duct to the case fan, so it is in effect acting as the CPU fan. The 1kHz noise goes away completely when I drop the fan speed to 600RPM. This however requires BIOS settings which don't start the fan at all until Windows is booted. The BIOS-regulated voltage is too low to start this fan and it is only once a Gigabyte-supplied utility program takes over and ramps the voltage to about 75% that the fan starts successfully at about 600RPM.
 
Does anyone know the minimum voltage the BIOS puts out on the CPU fan header when it's auto-regulated? The problem is that the 800RPM fan only gets going reliably at about 75% of it's rated voltage / speed. Would a faster-rated fan have a bigger operating window, say from 50% to 100% of it's rated voltage / speed, and therefore be more suitable?

I think you might have gotten a bad fan.  I have 2 of the 1600RPM S-Flex fans and the only thing I hear from them is air moving.

Sounds like a bad fan or possibly mount vibration. I am running 4 of the 1200 rpm 120s in my P182, no audible fans from my normal seating position.

4 Scythe S-Flex SFF21Fs regulated to 1200rpm. Nothing but a quiet woosh of air from a very noisy Stacker case. I can hear board/graphics card coils whine over the fans.