Another 92mm fan basically, which won't be particularly quiet. I suppose swapping it for a slower 92mm fan, might lower noise a bit but would have an effect on temperatures.
You might want to look into why the fan is noisy. Typically in an idle enviornment your cpu fan wont need to be spinning at max speed and can be throttled down by your motherboard's PWM controller.
You should also check it though. Reach in with your finger and stop the fan from spinning and see if the noise stops.
I'm not sure if you can strap a 120mm fan to the arctic heatsink as this may waste a great deal of the airflow around the edges.