Basic interaction mechanism between the air flow and viscoelastic mucus layer lining a rigid tube is computationally studied. Linear wave instability theory is applied to the coupled air-mucus system to explore the stability of the interface. Primary velocity profile is taken to be the mean profile of turbulent flow and turbulent fluctuations are neglected. The model predicts that the instability initiates in the form of slow propagating waves on the mucus surface. Onset flow speed at which these waves initiate is very sensitive to mucus viscosity to elasticity ratio at lower range and it approaches to an asymptotic value for higher values. The results indicate that while the wave length increases, wave speed decreases with increasing mucus viscosity to elasticity ratio. Model also predicts that the waves initiate at lower flow velocities for the turbulent case compared to the published laminar case. Turbulent onset flow speed is only 34% Flow is considered to be turbulent during forced expiration and coughing in central and upper airways. Model predicts that this flow behavior tends to favor wave initiation at lower flow rates and may facilitate cough clearance.