The unsteady behaviour of ventilated and natural cavity flows over a 2-D wall-mounted fence is investigated for fixed length cavities with varying free-stream velocity using still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities. Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, rather irregular small-scale shedding at the cavity closure. Small-scale cavity break-up was associated with a high-frequency broad-band peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step. A low-frequency peak reflecting the oscillations in size of the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependence).