In this study, the natural frequency changes, induced by vibratory stress relief, were analysed. Vibratory stress relief (VSR) is a possible alternative to thermal stress relief (TSR), which can be applied to welded structures and castings to reduce residual stresses. In this paper, acoustic-based natural frequency measurements will be presented to investigate the effect of VSR treatment. Natural frequency measurements were taken on large disc-type grey cast iron components, before and after the VSR treatment and we have analysed the results for five similar castings from the same batch. The natural frequencies change when the stress state of the workpiece changes. Changes in the frequencies have been examined, and we have identified a pattern for a specific treatment parameter setting. Measurement images – spectrograms - were generated to visualize the frequency response, where stress-induced variations appeared as shifts in resonance bands. Across all five tested components, a consistent decrease in dominant natural frequencies was observed. While the results mostly align with theoretical expectations, they also highlight the complex, time-dependent behaviour of residual stress redistribution. Further long-term monitoring is recommended to clarify post-treatment evolution and material-specific responses. The results can open new directions for the automotive and heavy industry to effectively reduce residual stresses and monitor the performance of the workpieces.