Mains frequency noise components
If the question is why do you see harmonics of the line frequency popping up as EMI in your measurements of a transfer function the the answer follows:

Harmonics on the AC line are generally caused by nonlinear loads.  For example, a bridge rectifier followed by a capacitor filter will cause the following kind of current waveform, as current flows only when the line voltage exceeds the capacitor voltage:



In many parts of the world regulations require power factor correction, or more precisely harmonic elimination circuitry to mitigate (but not necessarily eliminate) the problem, but lower power (&lt;75W for example) equipment is exempt.  So in aggregate this equipment can still cause harmonic distortion on the lines.

The IEC standard that applies is IEC 61000-3-2, which specifies the maximum allowable value for harmonic currents from the second harmonic up to the 40th harmonic current.