So it's probably nothing to do with the modified PSU If it was dropping to 4.3V then obviously it was having a little difficulty supplying the required current, either that or there was a voltage drop due to resistance in the lead supplying it

12V will be low unless you bypassed the internal regulator, i'm assuming you did this or you'd end up with about 9V to the audio output section which certainly won't help things...

I'm at a loss tbh, i do have a Caiman but i haven't played about with it yet.

What i'd suggest is you take the thing back to as near standard as possible & find out through experimentation which change introduced the noise/whine