  We investigated the snapshots of five NICER observations of the black hole
transient GX 339-4 when the source transited from the hard state into the soft
state during its outburst in 2021. In this paper, we focused our study on the
evolution of quasi-periodic oscillations (QPOs) and noise components using
power-density spectra. In addition, we derived hardness ratios comparing count
rates above and below 2 keV. The evolution from the hard to the soft state was
a somewhat erratic process showing several transitions between states that are
dominated by top-flat noise and can show type-C QPOs; those that are dominated
by red noise and can show type-B QPOs. From the parameters that we studied, we
only found a strong correlation between the hardness ratio and the type of QPO
observed. This implies that the appearance of type-B QPOs is related to a
change in the accretion geometry of the system that also reflects in altered
spectral properties. We also observed that the type-B QPO forms from or
disintegrates into a broad peaked feature when the source comes out of or goes
to the hard-intermediate state, respectively. This implies some strong
decoherence in the process that creates this feature.
