I have to apologize to Asus. Turns out I was wrong. It was a the rear exhaust fan making the sound. Possibly ball bearings wearing out.
Never heard it before because I switched to a Corsair H100i cooler and no longer have a big heatsink with a big fan masking out the noise.
As far as the weird temp reading. I have no idea.
Thanks but I would still like to use PWM mode in future. This is a major annoyance.
Noise is a natural byproduct of pwm (Pulse Width Modulation) Rather than vary voltage, full voltage is supplied to the fan but switched on & off really fast. The pitch of the frequency you hear is directly related to how fast that switching occurs (slower switching/pitch = faster running fans). Yes indeed, solid state components can produce noise - particularly pwm circuits. Most newer motherboards also use pwm in their power regulation circuitry as well and almost all produce a high-pitched noise even with no fans connected.
Some people made fun of me for still using fan controllers instead of the motherboard's headers by saying PWM would be quieter. WHY DID THEY LIE TO ME? I loved my fan controllers.