I'd say the most likely culprits are the CPU fan or the Hard Drive.
Maxtor makes loud/fast hard drives. I have one now and it's usually the only thing I can hear from my case besides the air passing through my 20 dba case fan. In the future if you want a quiet comp buy a Seagate hard drive, which runs slightly slower but much quieter.
If your hard drive is the problem you can look for some hard drive silencers (newegg.com or xoxide.com). If you find one you think is adequate, tell me, I haven't.
If it is your CPU fan you could buy any number of fans to replace them, just look at the dba. However keep in mind that a quieter fan has a slower RPM, thus less airflow, and if you are running a real powerful system or especially if you overclock, this will be a serious problem.
