Due to the default voltage regulation on the fan, a low RPM value for the fan was created by the BIOS so as to prevent noise.
Accelero Twin Turbo is using 2 ultra quiet PWM fans which allow optimal air flow rate but operate at an extremely low noise level.To achieve the best result, it is recommended to use RivaTuner for controlling the fan speed.
The latest RivaTuner can be download from here(by Guru3D.com)
Step 1
Download and install the RivaTuner from here(by Guru3D.com)
Step 2
Run RivaTuner
It will take a few seconds to scan your hardware in the first run. Then, you can see the main menu as below:
Step 3
If you have more than one display, select the target in the drop down menu
Step 4
Select the VGA card logo for "Low-level system setting" by clicking on the downward arrow next to the "Customize..."

Step 5
Select the "Fan" tag and check the box of "Enable low-level fan control"
Step 6
Select "Fixed" and then change the % of fan speed by adjusting the bar
It is recommended to use around 70% for achieving the best noise-performance ratio. You can also adjust the % for different fan speed according to your preference.
Step 7
Check the box of "Apply fan settings at Windows startup" to make sure that this setting will be activated every time you boot up the PC. 
Step 8
Finally, click "OK" to confirm all settings
You can now enjoy the Silence and the Ultimate Power
