Linear, noise-generating systems in electronics are usually described by mathematical techniques which were originally designed for linear, time-invariant systems, e.g. output noise spectra are computed by application of the Fourier-transform to the autocorrelation function of the output signal. Recently, it was pointed out that this description might be incomplete, since the parameters of noise-generating systems fluctuate, i.e. they vary with time. Therefore, rather than using the theory of time-invariant systems, the theory of linear time-variant systems should be applied. This theory will be adapted to electronic circuits which are fed by noise, and which have inner niose soruces. It will be applied to simple electronic circuits like a simple equivalent circuit of an integrate ohmic resistor, in order to show the effect of the time-variance of parameters to the noise spectrum.