We theoretically consider the spontaneous oscillations of an elastic cylinder in a streaming fluid flow. Such oscillations are responsible for stall flutter in e.g. turbine blades and bridges, or Æolian tones when in the acoustic range. The process of vortex separation from the oscillating surface is modelled as a selfexcited oscillator. The vortex separation frequency remains synchronized with the oscillations of the cylinder over a great range of frequency mismatches, enabling the amplitudes and frequencies to be calculated. Only when the vortex separation frequency is much less than the cylinder’s natural oscillation frequency the synchronization breaks down, and then two-frequency oscillations (beats) occur.