One end to USB ground an the other to measurments ground plane, solid aluminum plate 100x50cm that is grounded.
streamer is ungrounded and isolated on the aluminum plate.
I make no claim that the measurements are correct in absolute terms, but in relation to each other.
Ah - OK, I see a problem then. Have to go out now but will describe the problem as I see it (fwiw) later.
EDIT - not so much a problem as an explanation as to why this is not relevant for the audio performance of the streamer or downstream components.
Disclaimer - I've made some assumptions about your test setup which might invalidate the discussion below if they are wrong.
I've I understand correctly - you are measuring the difference between noise on the streamer ground and on a separate ground plane - the two have no reference to each other (you say the streamer is isolated from the measurement plane). I'm guessing (assumption 1) you are grounding the external plane to mains earth - you might as well say that the earth plane has the noise on it (it is shared with all the power devices in your building). What for example would you measure on that plane if you referenced it to a copper rod hammered into the ground.
Whatever noise in the streamer relative to that plane is irrelevant for the audio performance.
Looking in more detail - you have a common mode noise source (referenced to the earth plane), on an Ethernet cable, and are connecting that to the streamer. The only coupling mechanism (assuming (2) unscreened Ethernet cables) is via the parasitic capacitance in the Ethernet transformer. This is probably of the order of 6-7pF so is going to be very high impedance at audio frequencies.
Does quick calculation....
That is just over 1Mohm at 20kHz between the noise source, and the streamer ground plane. Impedance is higher at lower frequencies.
(note - with that level of coupling impedance - it is quite possible that your scope probes are influencing the measurement.)
But the streamer has no current path to the earth plane - so no current through the impedance - so the full noise voltage will be added to the streamer ground plane.
I think in your measurements you are getting around 400mV of noise on the streamer ground plane (relative to the earth plane).
For the audio system this is is of no relevance whatsoever. What matters is voltage differences between different parts of the audio circuit. So lets look at that.
Either the ground plane of the streamer will be earthed by connection to an earthed component downstream (EG DAC, preamp, amp)
Or - they will all be non earthed.
If they are are non earthed, then the non earthed grounds of al the devices will be connected together, there will be no ground current, and all grounds will be at the same potential - no impact on audio.
If any of the components are earthed, then there will now be a low impedance path back to your measurement earth plane via the interconnect. Lets say that path is as high as 1 ohm at 20KHz.
So now you have a 400mV noise source, a 1Mohm or higher noise source impedance, and 1ohm to your earth plane. So now you are going to have 400mv*1/1000000 = 400nV on the ground plane. This is not going to be an issue even if the full noise voltage is applied to the analogue output of the DAC into the amplifier (it is at -130dB compared with a 2V signal)