Transmission1

sounds like poor decouppling <<cant spell or do i care
try running a wire from the neg to chassis m8

That's some cap lol, imagine putting that reverse polarity by mistake, it'd take half your house away!

Wire from neg to chassis actually makes it worse for some reason?

So the switch mode psu creates this noise whether it's in the Murphy case or out, but doesn't create it with other rigs.
A separate external (linear) psu doesn't create any noise at all though, so it looks like that's the route I'm going to have to go. Add a 12v input socket on the rear of the case. Shame to do that but if it makes it useable then so be it

Sometimes switch mode psu's will whistle when there isn't enough load on them.

Try connecting an indicator bulb across the psu and see if that quietens it down.

If so, it might need a permanent load of about 1 amp on it to stop it howling.

A lot of the cheap psu's on eBay are designed to have about full load on them all the time (LED lights etc), so they don't bother too much with the no (or little) load conditions.

A lot of computer psu's are like that also.