KWU-PWR plants have shown a high neutron noise level which has caused operational problems. The region of interest is below 1 Hz, so thermal-hydraulic oscillations seem to be a cause of this high level. In the last years, the neutron noise has increased when the fuel elements design has changed. This may indicate that there is a relationship between the spectral characteristics of the neutron detector signals and the fuel elements behavior. In order to advance in understanding these phenomena, the S3K software has been used to simulate both mechanical and thermal-hydraulic perturbations. The simulated neutron detector signals were analyzed and compared with plant data.