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Properties of the gravitational perturbation force caused by a small inhomogeneity located on the surface of a neutron star are studied. The oscillating perturbation force in both the accretion disc rotating around the star and in the interior of a differentially rotating star is determined. Both vertical and radial components of the force are given and their relations are discussed. The frequency of the oscillations is given by the difference of the frequency of the rotation of the star surface, and the orbital frequency in the disc (the frequency of rotation of matter in the star interior). Outside the star, in the disc, the vertical and radial forces vary with the same phase. Inside a differentially rotating star, the variations of the forces are inthe opposite phase in an internal part of the star, while they are in the same phase in an external layer of the star. In an intermediate part of the star, an additional oscillatory change appears.