Aerodynamic damping is the key parameter to determine the stability of vibrating blade rows in turbomachinery design. Both, the assessments of flutter and forced response vibrations need an accurate estimate of the aerodynamic damping to reduce the risk of high cycle fatigue which may result in blade loss. However, only very few attempts have been made to measure the aerodynamic damping of rotating blade rows experimentally under realistic operating conditions, but always with friction damping being present. This study closes the gap by providing an experiment in which a turbine blisk is used to eliminate friction damping at the blade roots and thereby isolate aerodynamic damping. The blades are excited acoustically and the resulting nodal diameter modes are measured using an optical tip-timing system in order to realize a fully non-intrusive set-up. The measured vibration data is fitted to a SDOF-model to determine the aerodynamic damping. The results are in good accordance with the time-linearized CFD simulation. It is observed, however, that not only the sweep rate of the acoustic excitation, but also the variation of the rotational frequency during the sweep excitation, and the excitation frequency influence the apparent damping.