Great, problem solved! Look at all the things you've learned about interference and power supplies. Now, interference aside, you can check if it really can output 18V at 2A within the specified ripple and consider a fix to source of the 10MHz leak, as it could affect other instruments you have around.
Did turning of that Thunderbolt also get rid of the ~400Hz noise?
Yes it did. Some sort of beat against the 10Mhz somehow?
I would have thought that the power supply would have been shielded
enough when fully assembled, but I'm no RF expert.
Now I have no complaints about my $26 power supply :-)
Scott