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 .