  The relaxation of the collective quadrupole motion in the initial stage of a
central heavy ion collision at beam energies $E_{lab}=5\div20$ AMeV is studied
within a microscopic kinetic transport model. The damping rate is shown to be a
non-monotonic function of E_{lab} for a given pair of colliding nuclei. This
fact is interpreted as a manifestation of the zero-to-first sound transition in
a finite nuclear system.
