We have applied high-resolution neutron time-of-flight spectroscopy and ab initio lattice dynamics calculations to study the guest dynamics of La and Ce in the filled skutterudite system xFe4Sb12. Both the experiments and the calculation show that the guest dynamics is governed by collective vibrations, i.e. phonons. Unlike the case of localized rattlers the guest modes reveal dispersive character and a non-trivial Q-dependence of the signal intensity. The rather low-energy of the guest modes is due to their higher mass and weaker bonding with the host network.