The Corsair AX750 operates in fanless mode until the system draws more than 150 Watts from the combined DC output rails.
Since you don't have an i5-2500K that you can overclock to at least 4.7 GHz or higher the PSU is going to remain in fanless operation.
Try running Furmark to get your GeForce GTX 560 Ti to draw its maximum power (i.e. it should be over 210 Watts), then the PSU's fan should ramp up into quiet mode.
Once you exceed a power draw of 375 Watts then the fan will begin to ramp up above its quiet mode.
You can always add a second GeForce GTX 560 Ti in 2-way SLI and then run Furmark.
Thanks! My fan started spinning so its not broken!