  We have analysed RXTE archival data to study the aperiodic variability of the
black-hole binary GRO J1655-40 during the hard state of the 2005 outburst. We
computed the 0.008-64 Hz power spectral density during the rise and decay of
the 2005 outburst, and we found that they were reasonably well modelled by the
sum of two, broad Lorenztian functions in most cases (plus a narrow QPO) which
correspond to three different variability components. Our aim is to study the
evolution of the timing properties of the source during the outburst, by
studying the correlation between the characteristics of the broad-band noise
components in the power spectra and the source luminosity. Our results suggest
that the whole power spectrum shifts to high (low) frequencies as the source
luminosity increases (decreases), in agreement with previous studies of other
black hole binaries. However, we also detect a strong "hysteresis" pattern in
the "frequency-luminosity" plots, and show that the "critical" luminosity
limit, above which the timing properties of the source change, is different
during the rise and the decay phase of the outburst. We discuss the general
implications of these results in the context of the truncated disc model.
