The equation of motion method for the Green function is developed to describe collective excitations in the model of metallic rare-earth compounds taking into account the magneto-elastic interaction. Cubic compounds with a non-magnetic crystal-field ground state are investigated. A comparison with inelastic neutron scattering results for PrAl₂ in the ferromagnetic phase shows excellent agreement between the calculated coupled magneto-vibrational modes and the experimental spectrum. In the paramagnetic phase hybridised quadrupole-type exciton-phonon excitations can appear.