I have iMac27" with some problem on power supply unit.
The noise is terrible and unacceptable.
I open iMac and at the moment I have to try PSU stand alone to understand if is PSU problem or other boards connected to it.
Imac run correctly, power is correct but the noise is very terrible.
PSU is LITEON pn 614-0446 PA-2311-02A REV:B.
1. schematic pin out 16 pin connector
2. similar experiences in the world: it's very terrible to discover this terrible noise.