This paper deals with the results of an investigation of moulded cases such as acoustic deflectors housing noisy equipment of the vehicle. The main advantage of the deflector is to decrease the level of noise, but the phenomenon of self-excitation is important, too, because it becomes a new source of noise. Readjustment of the vibration modes spectrum is possible by creating special structural variations, which increase the internal dissipation of energy of the vibrations in the deflector material. This approach is related to the introduction of design variables, the kind of material or to the choice of the points of fixation. The principal indication of a vibration mode of such a structure as a deflector is the number of nodal points. This characteristic gives the possibility to select several groups of modes, which are effective from the viewpoint of decreasing vibration amplitudes. Other possibilities are related to the means of active vibroinsulation.An investigator has a possibility to change the configuration of the fixation bushing and the distribution of fixation points. For this purpose, the rheological properties of bushing and the deflector are considered. The properties of the material vary with the temperature, which increases not only under the effect of the surrounding equipment but also under vibrational excitation. The influence of the temperature for damping characteristics should be considered. Also the variation of durability loss of the structure which leads to the cracking of deflectors is important.