  We study the energy dependent photon variability from a thermal Comptonizing
plasma that is oscillating at kHz frequencies. In particular, we solve the
linearised time dependent Kompaneets equation and consider the oscillatory
perturbation to be either in the soft photon source or in the heating rate of
the plasma. For each case, we self consistently consider the energy balance of
the plasma and the soft photon source. The model incorporates the possibility
of a fraction of the Comptonized photons impinging back into the soft photon
source. We find that when the oscillation is due to the soft photon source, the
variation of the fractional root mean sqaure (r.m.s) is nearly constant with
energy and the time-lags are hard. However, for the case when the oscillation
is due to variation in the heating rate of the corona, and when a significant
fraction of the photons impinge back into the soft photon source, the r.m.s
increases with energy and the time lags are soft. As an example, we compare the
results with the $\sim 850$ Hz oscillation observed on March 3, 1996 for 4U
1608-52 and show that both the observed soft time-lags as well as the r.m.s
versus energy can be well described by such a model where the size of the
Comptonizing plasma is $\sim 1$ km. Thus, modelling of the time lags as due to
Comptonization delays, can provide tight constraints on the size and geometry
of the system. Detailed analysis would require well constrained spectral
parameters.
