  Aims: We identify the source of fast-drifting decimetric-metric radio
emission that is sometimes observed prior to the so-called flare continuum
emission. Fast-drift structures and continuum bursts are also observed in
association with coronal mass ejections (CMEs), not only flares. Methods: We
analyse radio spectral features and images acquired at radio, H-alpha, EUV, and
soft X-ray wavelengths, during an event close to the solar limb on 2 June 2003.
Results: The fast-drifting decimetric-metric radio burst corresponds to a
moving, wide emission front in the radio images, which is normally interpreted
as a signature of a propagating shock wave. A decimetric-metric type II burst
where only the second harmonic lane is visible could explain the observations.
After long-lasting activity in the active region, the hot and dense loops could
be absorbing or suppressing emission at the fundamental plasma frequency. The
observed burst speed suggests a super-Alfvenic velocity for the burst driver.
The expanding and opening loops, associated with the flare and the early phase
of CME lift-off, could be driving the shock. Alternatively, an instantaneous
but fast loop expansion could initiate a freely propagating shock wave. The
later, complex-looking decametre-hectometre wave type III bursts indicate the
existence of a propagating shock, although no interplanetary type II burst was
observed during the event. The data does not support CME bow shock or a shock
at the flanks of the CME as the origin of the fast-drift decimetric-metric
radio source. Therefore super-Alfvenic loop expansion is the best candidate for
the initiation of the shock wave, and this result challenges the current view
of metric/coronal shocks originating either in the flanks of CMEs or from flare
blast waves.
