Cheaper way is
you can tried to use the 12 volt fan and run it at 7 volt..
make sure its not your CPU fan..
url
hxxp://www.7volts.com/quiet.htm
My Tbird 1.2G o/c @1.33Gcome with a 60mm fan 7000rpm 2 ball bearing fan, moving air 38cfm, noise 46dba..
instead
I replaced it with a
80mm Ultra Quiet Fan, , 2700rpm sleeve bearing fan, 36.50cfm, 30dba.
The noise level drop ~ 14-16 dba..
if you have many fans..most likely your box sound like a 747 taking off
?