Replacing a loud fan in a 1RU server

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http://serverfault.com – I just bought a firewall that is very loud. The biggest problem is the two fans in the back (2 x Sunon 40mm fans) running at full speed. The firewall is 1RU with pfSense installed on it.The CPU is a Pentium 4 2.8 GHZ with a heat sink on top. The air is lead through a plastic tunnel across the unit with the fans in the back and the CPU in the front. Do i really need two fans running at full speed? Can I reduce it to something like 5v? Will that affect the performance or do I even risk burning the CPU? Are there any quiet fans I can replace them with? (HowTos)