You cannot use any other fans apart from identical cfm/rpm design due to the cooling. They are noisy because they need that much airflow over the heatsinks. Do you not think that if they could have used a slower quieter fan that it would be fitted as standard?! The whole noise/fan issue has been covered to death on the forum in various places. Please take a good look around and do some research.
Sorry i did't express myself clearly. These fans were meant to be as extra in both ends of the box.
They would actually cause more restriction. You want at least the same spec fans supplying and exhausting air. You could use acoustic ducting with an inline fan to help the exhaust. Again this has been covefed in the forum befofe numerous times.