Due to the demands for today's passenger cars regarding fuel consumption and emissions, exhaust turbo charging has become a fundamental step in achieving these goals. Especially in upper and middle class vehicles it is also necessary to consider the noise comfort. Today, floating bushings are mainly used as radial bearings in turbochargers. In the conventional operating range of the turbocharger dynamic instability occurs in the lubrication films of the bearings. This instability is transferred by structure-borne noise into audible airborne sound and known as constant tone phenomenon. This phenomenon is not the major contributor of the engine noise but its tonal character is very unpleasant.In order to gain a more detailed understanding about the origin of this phenomenon, displacement sensors have been applied to the compressor- and the turbine-side of the rotor, to be able to determine the displacement path. Also, part of this investigation is the measurement of the rotational speed of the floating bearing bushings on turbine-and compressor-side of the turbocharger. The investigations are carried out on turbochargers from 1.6l and 2.0l four-cylinder gasoline engines. The turbocharger has been decoupled from the internal combustion engine to separate the turbocharger related effects from engine related sources.The constant tone can be identified in both the structure-borne and the airborne noise of the turbocharger. At the beginning of the constant tone, during a ramp-up of the rotor, the amplitude of the shaft-movement increases on turbine-and compressor-side. At the same time, a high, jump-like increase in the bearing bushing speed is ascertained. For a detailed analysis, the signals from the displacement sensors are separated into their components, consisting of 1st order of the rotor and the sub-synchronous oscillations. It is shown that the proportion of 1st order in amplitude remains unchanged and the proportion of sub-synchronous oscillation increases significantly. This oscillation is transmitted by the bearing system to the turbocharger housing and emitted from there to structure-borne and airborne noise.Considering now only the sub-synchronous portion of the movement on the turbine-and compressor-side, not only an increase in the amplitude can be seen, but also a change in motion of the rotor at the start of the constant tone, from a conical into a cylindrical motion.