We report on a new method by which, for the first time, the viscoelastic properties of an adsorbed surfactant layer on a solid surface are measured. It is based on an analysis of the amplitude and the phase angle of the pressure fluctuations induced by a pulsating flow of a Newtonian surfactant solution through cylindrical pores. This method is subsequently used to determine the viscoelastic properties of an admicelle, formed when flushing surfactant solutions through nanopores, around the stick-slip transition. We find that the admicelle responds elastically for flow strengths below the transition and beyond the viscous. This is in agreement with the hypothesis formulated earlier.