Measurement of transformer with a vector network analyser (VNA)
Lets assume your noise floor is set by the termination resistors. These will n 50 ohms.

The noise density is 0.89 nanovolts/rootHertz; in 100Hertz bandwidth (a very slow frequency-sweep rate (so slow the VNA internal frequency synthesizer might set the noise floor), your nois floor will be sqrt(100) * 0.89nanovolt/rtHz

Or 8.9 microVolts  rns