  We present high time-resolution multicolour observations of the quiescent
soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the
secondary star's ellipsoidal modulation are large flares on timescales of a few
hours, as well as several distinct rapid flares on timescales of tens of mins.
The rapid flares, most of which show further variability and unresolved peaks,
cover shorter timescales than those reported in previous observations. The
power density spectrum (PDS) of the 5 s time-resolution data shows a
quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this
periodicity represents the Keplerian period at the transition between the thin
and advective disc regions, we determine the transition radius. We discuss the
possible origins for the QPO feature in the context of the advection-dominated
accretion flow model.
  We determine the colour of the large flares and find that the i' band flux
per unit frequency interval is larger than that in the g' band. The colour is
consistent with optically thin gas with a temperature of ~8000 K arising from a
region with an equivalent blackbody radius of at least 2 Ro, which covers 3
percent of the accretion disc's surface. Our timing and spectral analysis
results support the idea that the rapid flares (i.e. the QPO feature) most
likely arise from regions near the transition radius.
