Now on to voiding your warranty!
First SSH into the machine using your password:
Then edit the cgminer startup script.
There is a line that looks like this:
100 in there is the maximum fan speed
Now exit the editor
Like was said above it is recommended you use the -2 flag to voltage to avoid overheating if noise is of concern. Lower fan speeds means higher heat... lower voltage helps mitigate this.
Another option is put the device inside a box and use a dryer vent tube to exhaust the heat. Helps reduce noise by about 20db. Another is to grab cold air from outside. Lower heat will reduce fan speed even more. I had one Avalon running in a shed at 1200RPM for a while.
Good luck and remember I am not responsible for fires, destruction of property or warranty obstructions.