Hello Mr. Mjrtoo.
The 140mm fan speed in that PSU is driven by load and temp. Depending on how warm/hot the air supply rising from within the case might be, that could be adding to the thermal threshold, thus causing the PSU to initiate a higher fan speed to maintain cooling within the PSU itself, which is not totally the function of the PSU cooling elements themselves. It is VERY important to have good quality venting case fans on the rear, and even the case roof, to fully vent all that thermal energy that is being dissipated within the rig. Some cases and their equipment literally become mini-ovens when you crank up the unit.
I didn't specifically recall what all components you have in your rig, but in mine I have the N680i board, 4 x 1GB 1066 Crucial Ballistix memory sticks, E6850, 8800GTX, 2 x 150GB Raptors, 4 x Seagate 750GB, 3 x DVD-RW, Zip 750, floppy/card reader, ATI 650 TV tuner, 2 x PCIe cards, and a USR modem. I use a ToughPower 850 PSU, and it never gets much above warm, and the fan doesn't make much noise.
But, on the other hand, I don't run a game like FEAR, full-throttle probably like you might do, which is going to run power demands up against the wall damn quick. The fact that the PSU didn't crap out under that load is a credit to its integrity. However, quite frankly, and with all due respect, you can't reasonably expect to draw that kind of power from a PSU, and not have it exercise it's onboard cooling components - EXCEPT if you "Super-Size" the PSU, so that any "normal to above normal" load is just a mere blip on the radar screen of such an oversized PSU, and doesn't create enough of a load to cause that kind of a monster PSU to even break a sweat. In that case, I'm talking about something in the 1000 to 1200 range.
Now if you have dual video cards, especially 8800GTX models, then you are likely drawing between 150 and 175 watts for each of those cards alone, plus all the other stuff in your case. If you're anywhere near that level of wattage draw, then you may have undersized your PSU, if you want it to run near silent.
Aside from that, I don't have enough info to offer any other advice. Let us know what all the wattage draws are for your rig, and maybe we can help you further. But in any event, don't be disappointed, you made a good choice by purchasing a ToughPower 750 PSU. Best regards. TheBeagle 🙂