The results of experimental investigation of the relationship between low-frequency noise spectrum in an electric current through conducting filaments in Si3N4 films with thickness of 6 nm on n++-Si(001) conducting substrates and degradation characteristics of these films are reported. Two structures are investigated: (SN6) Si3N4/SiO2/Si, with 2 nm SiO2 sublayer between the film and substrate; (SN8) similar structure but without SiO2 sublayer. Detailed comparison of the structural parameters, such as average current through the filament, probability density function and spectrum, is presented with discussion of possible physical reasons for the difference between the testing samples and its effect on degradation characteristics.