  We report the results of an observation with the Rossi X-ray Timing Explorer
of the black hole candidate GX 339-4 during its 2002/2003 outburst. This
observation took place during a spectral transition from the hard to the soft
state. A strong (6% rms) transient quasi-periodic oscillation (QPO) appears
suddenly in the power density spectrum during this observation. The QPO
centroid is ~6 Hz, but it varies significantly between 5 and 7 Hz with a
characteristic time scale of ~10 seconds, correlated with the 2-30 keV count
rate. The appearance of the QPO is related to spectral hardening of the flux,
due to a change in the relative contribution of the soft and hard spectral
components. We compare this peculiar behavior with results from other systems
that show similar low frequency QPO peaks, and discuss the results in terms of
possible theoretical models for QPO production.
