  We study the timing properties of the bursting atoll source 4U 1728-34 as a
function of its position in the X-ray color-color diagram. In the island part
of the color-color diagram (corresponding to the hardest energy spectra) the
power spectrum of 4U 1728-34 shows several features such as a band-limited
noise component present up to a few tens of Hz, a low frequency quasi-periodic
oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise
component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to
these, in the lower banana (corresponding to softer energy spectra) we also
find a very low frequency noise (VLFN) component below ~1 Hz. In the upper
banana (corresponding to the softest energy spectra) the power spectra are
dominated by the VLFN, with a peaked noise component around 20 Hz. We find that
the frequencies of the kHz QPOs are well correlated with the position in the
X-ray color-color diagram. For the frequency of the LFQPO and the break
frequency of the broad-band noise component the relation appears more complex.
These frequencies both increase when the frequency of the upper kHz QPO
increases from 400 to 900 Hz, but at this frequency a jump in the values of the
parameters occurs. We interpret this jump in terms of the gradual appearance of
a QPO at the position of the break at high inferred mass accretion rate, while
the previous LFQPO disappears. Simultaneously, another kind of noise appears
with a break frequency of ~7 Hz, similar to the NBO of Z sources. The 100 Hz
peaked noise does not seem to correlate with the position of the source in the
color-color diagram, but remains relatively constant in frequency. This
component may be similar to several 100 Hz QPOs observed in black hole
binaries.
