If you cant really put your finger (or ear in this case) on it, then you could start removing parts to eliminate, one by one, to see which it is. Do this, of course, while the computer is turned off.
Its most likely a fan but could be a hard drive or Optical drive.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64