Does your mobo have a fan on the northbridge?
That, and your gfx card would be the first culprits I would have thought, followed possibly by your psu, and maybe your case fans (the Antec Tri-cools are alright fans, but not the quietest even on low).
I just recently got my P180 filled with Noctuas and it has never been quieter.
Noctua intake/exhaust for the case.
Noctua on a Scythe Katana 2 on the cpu.
Noctua on my Acellero on my 8800GT.
Passive mobo.
Enermax Noisetaker.
Seagate Barracuda drives.