This paper investigates the generation of rotor-alone tones and their contribution to the outflow noise in a transonic centrifugal compressor stage with vaneless diffuser and volute by means of unsteady full-annulus CFD-simulations. The aerodynamic field, as well as the generation and propagation of sound, were simulated simultaneously using the URANS-approach of the solver TRACE and a numerical grid consisting of 170M cells. To assess the accuracy of the predicted fluctuations, the investigation compares the simulated diffuser flow field to measured flow angles and pressure fluctuations obtained from experiments conducted on a large-scale test rig. The analysis explains the different sound generation mechanisms responsible for tonal components in the acoustic spectrum at the compressor outlet, based on the Fourier decomposition of the pressure fluctuations in diffuser and volute. Further, the paper analyzes the modal structure of the simulated sound field at the volute outlet by means of a radial mode analysis and discusses the influence of changing operating conditions on the sound power emitted. The analyses reveal that supersonic flow phenomena occurring at choked operating conditions cause a significant increase in noise emissions. Furthermore, the investigation shows that the sound field at the volute outlet is dominated by few low-order modes, a fact that justifies analysis using methods based on Compressed Sensing in future experimental investigations.