We considered of low-frequency perturbations of magnetic field in coronal loops when low-density proton and electron beams propagate in a loop. Two mechanisms of the generation of low-frequency perturbation of magnetic fields were analysed. The first mechanism is related to longitudinal current created by charged particle beams propagated in a loop. It is shown that such generation mechanism of Alfven waves can lead to low-frequency perturbations even at very low currents. This can promote the reconnection of magnetic fields and development of a flash. The second mechanism is not related to currents in a coronal loop. It is shown that for this instability proton beams excite instabilities at lesser proton beam densities than electrons. The increments and criteria of instability development are derived. Such instabilities can generate not only Alfven disturbances, but also kinetic Alfven ones.