UPDATE:

I have measured the voltage coming from the PSU. These are my results:

Under no load:

-Blue wire: -4.8v

-Yellow wire: 11.11v

-Orange wire: 5.27v

There is also a tick about twice a second-it seemed to speed up as the supply was running for a while.

Under load when powering mac:

-Blue wire: -5.06v

-Yellow: 12.3v

-Orange: 5.1v

The tick is not present when the supply is under load (I disconnected the fan and hard drive so I could hear if there was a noise)

In both cases, my meter has a 1M ohm impedance.