A theoretical model for the noise properties of Schottky barrier diodes in the framework of the thermionic-emission¿diffusion theory is presented. The theory incorporates both the noise induced by the diffusion of carriers through the semiconductor and the noise induced by the thermionic emission of carriers across the metal¿semiconductor interface. Closed analytical formulas are derived for the junction resistance, series resistance, and contributions to the net noise localized in different space regions of the diode, all valid in the whole range of applied biases. An additional contribution to the voltage-noise spectral density is identified, whose origin may be traced back to the cross correlation between the voltage-noise sources associated with the junction resistance and those for the series resistance. It is argued that an inclusion of the cross-correlation term as a new element in the existing equivalent circuit models of Schottky diodes could explain the discrepancies between these models and experimental measurements or Monte Carlo simulations.