The comparison of the phonon spectra of the parent MFe2As2 (M = Ba, Ca, Sr) and MFeAsF (M = Ca, Sr) compounds show a very strong renormalization in the lower and intermediate frequency part of the vibrations. Mass effects and lattice contraction can- not solely explain these changes. Further, the measurement of temperature dependence of phonon density of states across the magnetic/structural phase transition temperature indi- cates that softening of low energy modes in the paramagnetic state of these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. Detailed lattice dynamical calculations are carried out to analyze the experimental data.