Three-dimensional vortex dynamics in the wake behind a flat plate at 20^○ angle of attack are explored by means of the Hilbert–Huang transform. While a completely regular vortex shedding is observed at Reynolds number Re = 500 with a distinct shedding frequency and only a single subharmonic frequency, a complex shedding behavior is observed at Re = 525 and above. The low-frequency variations of the energy content and frequency in time are deduced from the Hilbert spectra. The low-frequency modulations along the span of the plate are substantially increased from Re = 525 to Re = 800. The mean marginal spectra reveal that the energy in the low-frequency band increases with increasing Re. A corresponding reduction of the energy content in the high-frequency band is observed at the highest Reynolds number.