If it's a Sonata Mk I with the TP-380S PSU, then you'll cut the noise noticeably by switching to something quieter. The change to the S12-430 was one of the top four quieting improvements for me. Others in the top four already mentioned:
- Active to passive chipset cooler (Zalman NB-47J). If your mb uses loops rather then push pins, and you don't want to make your own wire clip
, Zalman has just announced the ZM-NBF47 with hooks
- Active to passive graphics cooler (Aerocool VM-101
- Stock CPU cooler to Zalman 7000, then to Thermalright XP-120 w/ Papst fan
Other good changes for a Sonata-cased system:
- Improved isolation for the hard drives in the sleds with this method
created by m0002a. Cheap and easy if you have access to US-threaded screws.
- Replacing the front panel with same from a Sonata Mk II (better intake air flow; $20 direct from Antec)
- Quieter (Nexus, Yate Loon or Globalwin) 120mm fans for exhaust and intake (latter at lower voltage). The supplied Antec isn't bad, but these are quieter.
- Switch to a quieter HDD (Samsung SP2004C for me) for primary storage