Knocking noise is a transient structural noise triggered by piston rod vibrations in the shock absorber that excite the vibration of chassis components. Piston rod vibrations can be caused by valve motion (opening and closing) and dry friction during stroke inversions. This study investigates shock absorber knocking noise in twin tube gas-filled automotive shock absorbers and its aim is to define an acceptance criterion for a sample check of the component. If, in fact, the damper comes from a large mass production, it may happen that small mounting differences lead to different behaviors that result in higher or lower levels of knocking noise. To achieve this goal, experimental tests were carried out using a hydraulic test bench; accelerometers were placed in proximity to the rebound valve and on the piston rod. The vibration phenomenon was then isolated through a post-processing analysis and a damped and unforced lumped mass model was used to characterize the vibration. The study suggests the peak amplitude of the piston acceleration rod as a classification parameter.