what kind of fans do you have? what size? how long have you had these fans for? 80mm fans tend to be louder, so if you have several of those, it won't be very silent.
also what is the noise level(dBA) on them? the lower the better. try to find some that are 15-17 dBA fans for an ultra quite pc.
try these 120mm fans
they are REALLY cheap, and REALLY quite. great buy