Hi! Since you can tell it's too loud for your taste, I assume you've got the AR up and running? How about measuring the actual temperatures during high load? Then you could slow down the fan and see how that changes. You could do this either by lowering the voltage by a constant (say, with a zener), or making it adjustable (voltage divider and potentiometer). Or if you want to make it fancy do, pwm with an attiny or the teensy - you could adjust speed based on temperatures.