A1076 20" G5 iMac Identifying fans

I have recently repaired a blown capacitor on my old iMac and replaced the hard drive, but since doing so,

one of the fans suddenly sounds like a jet! I have ran the Apple Hardware Test and it reported: 2FAN 4/8 system fan

I know there is a left upper fan and a right upper fan and also a small lower intake fan, but which is

the test referring to? I've checked all over but cannot find any reference for system fan! Please help!