Time resolved excitation and detection of the coherent vibrational motion of metal nanoparticles are discussed in the light of femtosecond pump-probe experiments performed in silver nanoparticles with radius ranging from 2.1 to 15.3 nm. Analysis of the phase of the observed sample absorption oscillations shows that coherent excitation is dominated by an indirect displacive mechanism due to subpicosecond heating of the lattice by fast electron-lattice energy transfer for large nanoparticles (R > 10 nm). For smaller particles, the results suggest an additional contribution from direct coupling with the non-equilibrium electron gas. Both mechanisms, being related to an isotropic particle expansion, the fundamental radial mode is preferentially excited because of its better spatial matching with the excitation process. Optical control of the acoustic nanoparticles vibration is also demonstrated.