It could also be that the fans you have are just wearing out, you could always try to find replacements.
nextfan has the dB (noise) ratings on all the fans, normal case fans range from 30-50dB. but you can find fans that are as low as 9dB perhaps you could find a LED fan in the lower ranges.
this might be enlightening as well if you want to eliminate noise:
http://reviews.ebay.co.uk/How-to-build-Qui...000000001675454
I have 5 SCSI hard drives in 1 PC so they make a heck of a lot more noise than the fans my computer room sounds like a server room at a small company.