The spectral densities of the fluctuations of noise-driven underdamped nonlinear oscillators are discussed with particular reference to the large class of systems whose eigenfrequencies vary nonmonotonically with energy. It is shown by analog electronic experiments and theoretically that, astonishingly, the widths of their spectral peaks can sometimes decrease with increasing noise intensity T. The specific system studied, as an example, is the single-well Duffing oscillator in a constant homogeneous field. An explicit expression is derived in terms of T and a field parameter for the shape of the spectral peak. It is shown to be in good agreement with experiment. The possibility of observing such spectral features for localized and resonant vibrations of impurities in solids is discussed.