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The problem of the vibration induced instability of a liquid film flow is formulated, thanks to a linear approach. For the solution of the problem, the disturbances are expanded using a spectral method based on Chebyshev polynomials leading to a system of ordinary differential equations. This system is solved by using Floquet theory. The natural and vibration induced instabilities are studied in terms of frequency, temporal growth rate, and eigenfunction. Numerical results show that the mean flow velocity alters the surface wave frequency, which is not predictable by Mathieu's equation model (which assumes that the fluid is at rest). Above a threshold amplitude, multiple vibration induced instability modes are triggered. At specific amplitudes above the latter, two wavenumbers coexist at the free surface. A relation between the eigenfunction shape and the instability mode is observed.

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