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, Hα, 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-Alfvénic 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-Alfvénic 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.