One of the main problems arising with highly loaded transonic compressors near stability limit is the occurrence of safety critical mechanical blade vibrations prior to rotating stall. For one specific type of vibration, small scale flow separation mechanisms at the leading edge are responsible by creating a convective coupling between adjacent blades. The analysis of this excitation source is very demanding for numerics and experimental investigations. Despite the fact, that this type of non-synchronous-vibrations (NSV) has been observed by many investigators, the phenomenon is still not comprehensively understood. A measurement campaign with extensive aerodynamic and mechanical instrumentation has been carried out at a high speed compressor to analyze the occurring effects. It was possible to derive the chain of cause and effect leading to NSV during repeated transient maneuvers and to identify the flow structure and wave characteristics of the traveling disturbances. The understanding of this failure mechanism helps to understand why casing treatments are very efficient to increase the stability range of the compressor. The presented results will show the characteristic of the aerodynamic excitation and the resulting mechanical response for a representative case.