  In this letter we uncover a new parametric resonance of axionic cosmic
strings. This process is triggered by the presence on the string of internal
mode excitations that resonantly amplify the amplitude of transverse
displacements of the string. We study this process by running numerical
simulations that demonstrate the existence of this phenomenon in a $(3+1)$
dimensional lattice field theory and compare the results with the analytic
expectations for the effective Lagrangian of the amplitude of these modes and
their interactions. Finally, we also analyze the massless and massive radiation
produced by these excited strings and comment on its relevance for the
interpretation of the results of current numerical simulations of axionic
cosmic string networks.
