  We report the observation of an intense anomalous peak at 1608 cm$^{-1}$ in
the Raman spectrum of graphene associated to the presence of chromium
nanoparticles in contact with graphene. Bombardment with an electron beam
demonstrates that this peak is distinct from the well studied D$'$ peak
appearing as defects are created in graphene; the new peak is found non
dispersive. We argue that the bonding of chromium atoms with carbon atoms
softens the out-of-plane optical (ZO) phonon mode, in such a way that the
frequency of its overtone decreases to $2\omega_{\rm ZO}\sim\omega_{\rm G}$,
where $\omega_{\rm G}$=1585~cm$^{-1}$ is the frequency of the Raman-active
E$_{\rm 2g}$ mode. Thus, the observed new peak is attributed to the 2ZO mode
which becomes Raman-active following a mechanism known as Fermi resonance.
First-principles calculations on vibrational and anharmonic properties of the
graphene/Cr interface support this scenario.
