This paper provide the microscopic phenomenological theory of analysis for the problem of nonequilibrium fluctuations, i.e. the statistical theory for description of nonequilibrium fluctuations. This theory is based on a generalized nonequilibrium entropy and an extension of the Einstein formula for the probability of the fluctuations. We obtain the second moments of nonequilibrium fluctuation of the specific energy and the heat flux in rigid heat conductors by calculation. This approach leads to nonequilibrium corrections to the conventional fluctuation-dissipation expressions for the heat fluctuations. The corresponding numerical corrections are obtained for phonon heat transport in dielectrics and for electronic heat transport in metals.