Substantial progress has been achieved in the identification of loose parts which had been detected by acoustic monitoring of reactor primary system. Several years of practical experience and the use of the offline digital analysis system MEDEA proved that acoustic monitoring is very successful for detecting component failures at an early stage. ISTec is involved in loose parts monitoring in several nuclear power plants in Germany. Advanced powerful tools for classification and evaluation of burst signals have been realised.Loose parts monitoring systems, which are installed in all German nuclear power plants (NPPs), indicated specific impact conditions at lower plenum of two BWR's. Flow tests were carried out with various coolant flow rates of internal axial pumps and use of model nuts in one case. More than 2000 different bursts have been analysed to provide information in detail about impact occurrences, their spectral characteristics and impact sequences. Burst shape parameters could be determined and signal amplitudes have been trended. Determination of the sound origin — fixed origin in one case, flow-induced moving origin in the other case - and mass estimation of the loose parts could be performed by application of advanced burst analysis methods. Characteristics of the impact signals are presented in the paper.