The established description of linear, noise-generating systems in electronics uses mathematical techniques which were originally designed for linear, time-invariant systems. Noise spectra are often extracted from model descriptions in the time-domain 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, the theory of linear time-variant systems (LTV-systems) should be applied rather than the theory of time-invariant systems. This was done for a very simple linear two-pole system. It turned out that even in this simple system novel, unexpected parts of the spectrum appeared in the output signal. In the submitted paper, this theory is expanded to linear, noise-generating four-poles and other n-ports. The mathematics of LTV-systems are well known since the fifties. It appears, however, as if their application to fluctuating noise-generating systems were limited to the analysis of narrow-band noise in the transmission channels of radio-links. The theory will, therefore, be adapted to electronic circuits which are fed by noise, and which have inner noise sources. It will be applied to a simple model of an integrated ohmic resistor in order to show the (so far unexpected) effect of the time-variance of parameters to the noise spectrum.