Predicting unsteady aerodynamics and related phenomena in the vicinity of the stability limit of transonic compressors has been a challenging topic within the turbomachinery community since decades. To improve numerical tools and prediction capabilities the Transonic Compressor Darmstadt Open Test Case has been introduced to provide a reliable validation test data set for steady and unsteady compressor aerodynamics. This work investigates the unsteady aerodynamics before the occurrence of rotating stall within the TU Darmstadt OpenStage compressor rotor and introduces unsteady wall pressure data to the Darmstadt Open Test Case. Exemplary pre-stall aerodynamics and stall inception are investigated for nominal (transonic) and part speed (subsonic) operating conditions, comprising the analysis of the unsteady static pressure field at the rotor tip as well as spectral analysis of the corresponding pressure signals during transient throttling maneuvers. To capture the rotor tip flow field, the rotor casing is instrumented with axially arranged flush mounted time-resolving wall pressure transducers. To determine propagation speeds of occurring phenomena, additional circumferentially distributed sensors are considered. Vibration monitoring shows no significant flutter or non-synchronous blade vibration, prior to the occurrence of rotating stall. Thus, this work focuses on aerodynamics, such as the interaction of secondary flow and shocks.