The Raman spectroscopy, involving phonons with nanometric wavelengths, is a powerful technique for investigating nanostructures. It evidences the localisation or mixing effects associated with the electronic states. On the other hand, the spatial coherence of the vibrational modes, at the origin of Raman interferences, allows to probe both electronic and acoustic properties. The approach developed in this manuscript lies in the comparison between measurements and scattering cross section calculations. It gives a quantitative understanding of both frequencies and intensities of observed Raman features. This approach allowed to identify a new coupling mechanism between plasmons and phonons which indeed dominates the low-frequency Raman scattering in metal nano-particules.