Another suggestion: Run it at a lower voltage. Yes, less CFM, but there is a point where you'll notice a significantly lower noise floor, while still getting enough air through the unit to keep it cool.
I did this with a Sony "HD" radio tuner... it ran QUITE HOT until I installed a 40cm 12V fan into the cover. I ran the fan at 5v. It works, is very quiet, and keeps that unit *MUCH* cooler than no fan.
You could try a resistor in series with the 12v fan supply, a 7808 regulator, or tap into the 5V supply. Add a 100uf cap "at the fan" to smooth out any noise issues it might cause.