The first unit of the Paks nuclear power station has been equipped with a noise analysis system. Spectral characteristics of signals from SPN detectors, ionization chambers, pressure and vibration transducers are regularly evaluated by FFT and AR analysis. The measured spectra are being interpreted in terms of noise sources and transfer functions using different empirical and theoretical approaches. The mathematical modelling of reactor primary system vibrations, the stochastic model of neutron flux fluctuations induced by control rod movements and a description of the in-core temperature noise effects were used. During the measurements in the start-up phase the effects of different operating conditions were investigated. In stationary reactor operation the long-term stability of the spectra are studied. Examples of measurement results are shown with a discussion of interpretation, and the possible diagnostic value of these investigations is examined.