The grazing flow of a fully turbulent boundary layer along a perforated plate, which is bounded by a closed cavity on its backside, can give rise to highly coherent, self-sustained oscillations of the shear flow, even in absence of acoustic resonant or fluid elastic effects. These oscillations are characterized in terms of unsteady pressure fluctuations and quantitative images of the instantaneous and averaged flow structure using a technique of high-image-density particle image velocimetry. This purely hydrodynamic instability, which rapidly emerges above the turbulent background, has a wavelength that is much longer than the hole diameter of the perforated plate. Variations of the effective length L of the perforated plate show nearly invariant values of dimensionless frequency fL/U, in which f is the predominant frequency of oscillation and U is the freestream velocity. In fact, this relationship holds even when the diameter of the hole pattern is altered. Variation of the hole diameter D does, however, strongly influence the amplitude and degree of organization of the self-sustained oscillation. It is demonstrated that, as the hole diameter becomes larger relative to the inflow boundary layer thickness, the amplitude of the predominant spectral peak is substantially attenuated and, in a limiting case, undetectable. These features are interpreted in conjunction with instantaneous and averaged patterns of the flow structure, which include distributions of both Reynolds stress and amplitudes of spectral peaks. This type of instability is distinctively different from the classical instability associated with "singing" of perforated plates employed in attenuation liners in engineering systems, where the genesis of the instability is local vortex shedding within each hole of the perforation, in contrast to the long wavelength instability that is addressed herein. © 2003 Elsevier Science Ltd. All rights reserved.