Might be a stupid question...but have you increased the fan speed of the cooler to max (2000rpm) when under load? If that's too loud, you could consider replacing the default fan with a better fan that deliver higher airflow/static pressure at a lower fanspeed.
Also considering your case being Antec 900, it might be better to have the cooler blowing toward the top and use the case's top exhaust instead of rear.
Thanks, sounds like a good idea, will change the direction of the Tuniq and see if that helps.

