The surrogate schematic details are a little unfamiliar. Are those coax feedthroughs that are electrically isolated from the box on boththe center conductor and the shield?? Can you draw it differently? Is the 75 ohm resistor connected to the surrogate box? Is the box metal ? I assume so?
Are you going to use a signal analyzer that will actually identify the noise frequencies and power spectrum? Or just monitors?
I like the hummer idea.
What is common impedance coupling? Same as common mode noise?
The assumption below is that a properly designed device will have very low resistance between device chassis and ground contacts for each input or output, right?
4. Connect one hummer lead to the device chassis and touch the other lead to the ground contact of each input or output connector. If the device is properly designed, there will be no output hum or change in the noise floor.