Current fluctuations associated with the classical size effect, for which the mean free path of the carriers λ is comparable to, or greater than, the film thickness d, have been investigated. The Monte Carlo approach has been extended into the Knudsen regime of electron transport. Using this method, the autocorrelation function and the spectral density of the fluctuations depending on two parameters (the ratio γ=λ/d and the surface specularity p) have been calculated. A procedure to generate the angle of diffuse electron scattering at the surface is described for both the Fuchs and the Soffer boundary conditions. It is demonstrated for both models that, with increasing γ and with decreasing p, the low-frequency noise is suppressed, with a redistribution toward higher frequencies. In such a case, the autocorrelation function is not exponential and the corresponding spectral density of the fluctuations is no longer Lorentzian.