Instability that results in modulation of the neutron flux from the GOL-3 discharge is identified. It is shown that the flow of hot plasma through cold cells of a multiple-mirror trap can lead to excitation of oscillations in these cells with ~ Ti/[script l]. The frequency range and the parallel polarization of the oscillations ensure that they scatter most passing ions in parallel velocity, thus providing a self-sustaining mechanism of improvement of parallel confinement.