Due to manufacturing tolerance and deterioration during operation, fan blades in the aero-engine exhibit geometric variability. This leads to asymmetry in the assembly which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternative Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference, resulting in two groups of blades. This phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. This study focuses on a type of non-integral vibration which occurs during APD. After the formation of alternating tip stagger pattern, APDs unsteady effect can cause the blades from one group to switch to the other, creating a travelling wave pattern around the circumference.It was found from numerical assessment on a randomly mis-staggered assembly that real engines can potentially experience such travelling disturbance and suffer fatigue damage. An idealised case is used to capture the bulk behaviour from the more complex cases in real engines and to decipher the underlying mechanism of this travelling disturbance. The results indicate that the driving force originates from the interaction between passage shock displacement and the passage geometry.