  Based on the analysis of a small sample of BATSE and Konus gamma-ray bursts
(GRBs) with know redshift it has been reported that the width of the
autocorrelation function (ACF) shows a remarkable bimodal distribution in the
rest-frame of the source. However, the origin of these two well-separated ACF
classes remains unexplained.We complement previous ACF analysis studying the
corresponding power density spectra (PDS). With the addition of Beppo-SAX data
and taken advantage of its broad-band capability, we not only increase the
burst sample but we extend the analysis to X-ray energies. The rest-frame PDS
analysis at gamma-ray energies shows that the two ACF classes are not simply
characterised by a different low frequency cut-off, but they have a distinct
variability as a whole in the studied frequency range. Both classes exhibit
average PDS with power-law behaviour at high frequencies (f' > 0.1 Hz) but
significantly different slopes, with index values close to those of Brownian
(-2) and Kolmogorov (-5/3) spectra for the narrow and broad classes
respectively. The latter spectrum presents an additional PDS component, a
low-frequency noise excess with a sharp cut-off. At X-ray energies we find the
power-law index unchanged for the broad class, but a significantly steeper
slope in the narrow case (~ -3). We interpret this as an indication that the
broad class bursts have weaker spectral evolution than the narrow ones, as
suggested also by our analysis of the ACF energy dependence. The low and high
frequency PDS components may then arise from two radiating regions involving
different emission mechanisms.
