common-mode noise in single-ended parallel wire
So as shown in the figure below, a transmission line composed of two parallel wires above a ground plane, there will be common-mode noise of 8V coupled into ground.


If the 8 V is constant, then this isn't common mode noise. It's a common mode bias.

If the 8 V fluctuates randomly, then the that fluctuation is common mode noise. 

For example if there is a 100 mV rms fluctuation on top of the 8 V, then you have 8 V common mode bias voltage, and 100 mV rms common mode noise. 


  can single ended transmission line have common-mode noise (coupled into the ground plan in this case)?


If the return conductor is only weakly earthed (for example, connecting it to earth through a 1 megohm resistor is fairly common), or if it's only earthed at one point, and there are random stray magnetic fields through the loop between the transmission line and earth, then you will see the return line potential (at different points along the line) fluctuate around 0 V, which will contribute to common mode noise.

You might also see this behavior called a ground loop.