  Employing Unruh's analogy to gravity, we study sound propagation in
stationary and locally irrotational vortex flows where the circulation is wound
around a long (rotating) cylinder. Apart from the usual scattering solutions,
we find anomalous modes which are bound to the vicinity of the cylinder and
propagate along its axis -- similar to whispering gallery modes. For supersonic
flow velocities (corresponding to an effective ergoregion), these modes can
even have zero frequency. Thus they should be relevant for the question of
stability or instability of this set-up.
