Electron density oscillations in a linear RF discharge operated with helium as the filling gas are measured by means of a 4 mm microwave interferometer, the microwave beam passing the discharge along a diameter. The azimuthal wave numbers and the direction of rotation are determined from ICCD camera images which are shot end-on. The wave number parallel to the magnetic field is obtained from the phase relation of modulated light emission received side-on at different axial positions. The oscillation frequency is typically lower than both the ion cyclotron frequency and the frequency of momentum transfer collisions between ions and neutrals. A sharp onset of oscillations accompanied by a weakening of plasma confinement is observable provided some critical magnetic field strength has been exceeded.