High amplitude levels of blade vibration have occurred on the first rotor of a multi stage high pressure compressor. The frequencies are not in resonance with harmonics of the rotor speed. The excitation is aerodynamically caused and associated with a rotating flow instability in the blade tip region of the first compressor stage. A vortex shedding mechanism can be interpreted as a rotating source to generate pressure waves. The source moves relative to the blade row at a fraction of the rotor speed, similar to the 'well known mechanism' of rotating stall. To investigate the unsteady flow field in the tip region of the rotor and its relation to the blade vibration, measurements of the pressure and velocity fluctuations in the vicinity of the blade tips are compared with blade vibrations. A calculation model of the spectral characteristics of the pressure fluctuations confirms the measured data. The effect is sensitive to aerodynamic blade loading so that a modification in the design could reduce the mechanism of the rotating excitation.