The present work analyzes the light-induced motion of the metal colloidal nanoparticles, aggregated into small clusters (≲ 10 monomers). Within the model of a viscous solvent the characteristic mechanical response time is found. The calculations in the approximation of the weakly-bound particles (that means, when the electromagnetic energy of the interacting induced dipole moments significantly exceeds the potentials of the other forces in colloidsteric, dispersion ones, etc.) show the value of the cubic nonlinearity to be χ(3) ∼ 10-10 esu and more, at the typical response time of ∼ 10-10 - 10-9 sec. Thus, the light-induced shift of the colloidal metal particles can produce a large optical nonlinearity of the colloids, at least for nanosecond light pulses.