  We study dynamical response of a nonlinear dimer to a symmetrically injected
monochromatic wave. We find a domain in the space of frequency and amplitude of
the injected wave where all stationary solutions are unstable. In this domain
scattered waves carry multiple harmonics with equidistantly spaced frequencies
(frequency comb effect). The instability is related to a symmetry protected
bound state in the continuum whose response is singular as the amplitude of the
injected wave tends to zero.
