  Dynamics of linear perturbations in a differentially rotating accretion disk
with non-homogeneous vertical structure is investigated. It has been found that
turbulent viscosity results in instability of both pinching oscillations, and
bending modes. Not only the low-frequency fundamental modes, but also the
high-frequency reflective harmonics appear to be unstable. The question of the
limits of applicability of the thin disk model (MTD) is also investigated. The
insignificant distinctions in the dispersion properties of MTD and
three-dimensional model appear for wave numbers k<(1-3)/h (h is the
half-thickness of a disk). In the long-wavelenght limit, the relative
difference between eigenfrequencies of the unstable acoustic mode in the
3D-model and the MTD is smaller than 5%.
