A container of rectangular parallelepiped form filled by ideal incompressible fluid is studied. The container is closed by an elastic cover which is modeled by a membrane or by a plate of constant thickness. The rest container sides are undeformable. The frequency spectrum of small free vibrations of the cover is built. The motion of fluid is assumed potential and the attached mass of fluid is taken into account. The main peculiarity of problem is that the fluid volume under cover is not changed. As a result the mode of cover deflection satisfies to a restriction equation which follows from the condition that the fluid volume under cover is constant. Refs 11. Figs 5.