  We present the results of timing analysis of the low-frequency Quasi-Periodic
Oscillation (QPO) in the Rossi X-Ray Timing Explorer data of the black hole
binary XTE J1550--564 during its 1998 outburst. The QPO frequency is observed
to vary on timescales between $\sim$100 s and days, correlated with the count
rate contribution from the optically thick accretion disk: we studied this
correlation and discuss its influence on the QPO width. In all observations,
the quality factors ($\nu_0$/FWHM) of the fundamental and second harmonic peaks
were observed to be consistent, suggesting that the quasi-periodic nature of
the oscillation is due to frequency modulation. In addition to the QPO and its
harmonic peaks, a new 1.5$\nu$ component was detected in the power spectra.
This component is broad, with a quality factor of $\sim$0.6. From this, we
argue what the peak observed at half the QPO frequency, usually referred to as
"sub-harmonic" could be the fundamental frequency, leading to the sequence
1:2:3:4. We also studied the energy dependence of the timing features and
conclude that the two continuum components observed in the power spectrum,
although both more intense at high energies, show a different dependence on
energy. At low energies, the lowest-frequency component dominates, while at
high energies the higher-frequency one has a higher fractional rms. An
interplay between these two components was also observed as a function of their
characteristic frequency. In this source, the transition between low/hard state
and hard-intermediate state appears to be a smooth process.
