In an effort to make your PC more quiet, the first thing you need to do is pinpoint the biggest source of the noise. To do this, turn your PC off and unplug it. Then, unplug all of your case fans. DO NOT unplug your CPU fan or any motherboard fans such as a chipset fan. Then, plug the PC back in and turn it on. Use your trusty ears to then judge the level of noise without any case fans running. If you, then, determine that the CPU fan is the largest source of noise, then you know right then and there that you can purchase a quieter CPU fan and quiet the PC down quite a bit. Otherwise, power the PC down again and plug in one case fan and restart. Repeat this for each fan of the PC, noting the level of noise. When you are done, you should be able to pinpoint which of the case fans is the loudest and hence which is the best candidate for replacement.