We report on the first experimental observation of transient free-carrier absorption-detected gratings in highly-luminescent porous silicon membranes. The sloping changes on 400 mu s long decay scale are observed below the injection onset for one excited electron-hole pair per nanocrystallite which are related to a notable shortening in the microsecond recombination form. A preliminary microscopic model for the dispersive recombination is suggested including the photoinduced fraction of uncorrelated carriers, which recombine according to a power decay, in contrast to the excitons which recombine according to a stretched exponential. At low excitation, we extract the upper limit of exciton diffusion coefficient and diffusion length as D = 3.4 X 10(-5) cm(2) s(-1) and L = 315 nm.