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