  Since the beginning of the Large Hadron Collider (LHC) commissioning,
spectral components at harmonics of the mains frequency (50 Hz) have been
observed in the transverse beam spectrum. This paper presents an overview of
the most important observations, collected during the latest physics operation
of the LHC in 2018, which clearly indicates that the harmonics are the result
of a real beam excitation rather than an instrumental feature. Based on these
findings, potential sources of the perturbation are discussed and a correlation
with power supply ripple originating from the magnets' power supplies is
presented.
