[en] Neutron noise is used for monitoring internal structures: neutron noise signals contain vibratory information concerning several components (core barrel, cylindrical thermal shield, fuel assemblies...). Analysing one particular mode of each component allows to know its fatigue condition and to detect a possible failure. Such a surveillance is sometimes difficult to perform: the modes may occur at very close frequencies and be mixed (the core barrel swinging mode and the thermal shield 2. shell mode for instance). Methods of signal processing have been performed in order to separate the modes or extract the useful mode. The paper presents these methods and the results obtained on the 900 MW PWRs. (author)