The prevailing view of the dynamics of flapping flags is that the onset
of motion is caused by linear instability of the initial planar state. This
view is reexamined by considering the forced motion of a flag immersed in a high-Reynolds number flow and subject to vortex shedding from its
cylindrical pole. Vortex shedding is represented by a `street' of discrete
line vortices released periodically from the pole and convected in the mean
wind over the surfaces of the flag. It is found that forced motion is
possible when the flag is still temporally stable, which suggests that the
present mechanism should be taken into account in future high-Reynolds
experimental investigations. A linear theory is then proposed for describing
the sound production of the flag in nominally uniform high Reynolds number
flow. Acoustic radiation of dipole type is calculated and discussed in the
limit where the flag is acoustically compact. It is found that the acoustic
pressure is dominated by the contribution from the trailing-edge wake and
that light flags are noisier than heavy.