differential mode noise

If you have a signal line referred to a common, and any voltage gets added (put in series) between them, that is a differential noise. One of the most common causes is the signal to common loop encloses changing magnetic field, which induces voltage in the loop. It is also possible for changing electric field that terminates on both signal and common lines to cause currents that produce different voltages due to different impedances in the path of each of the currents, and this will cause differential noise.

In power converter circuits, the magnetic and electric fields can originate in the converter, itself, and contaminate voltage reference or voltage or current measurement signals. More often, differential noise is caused just by the large current and rate of change of current in traces and planes that cause these conductors (that are often expected to be zero drop nodes) to have significant voltage differences between different locations due to resistance and inductance.