Brillouin and Raman spectra of a 0.71Pb(Ni1/3Nb2/3)O3-0.29PbTiO3 single crystal were measured in two light scattering geometries to elucidate the origins of central peaks that is related to the dynamics of the polar nanoregions. Two Lorentzian-type central peaks exist in the Brillouin spectra. The shape and the full width at half maximum of the broader peak in Brillouin spectra are almost same as that observed in Raman spectra. This indicates that two relaxations occur in the polar nanoregions. These two processes might be originated from the different switching directions of polarization.