  Gravitational waves offer a potent mean to test the underlying theory of
gravity. In general theories of gravity, such as scalar-tensor theories, one
expects modifications in the friction term and the sound speed in the
gravitational wave equation. In that case, rapid oscillations in such
coefficients, e.g. due to an oscillating scalar field, may lead to narrow
parametric resonances in the gravitational wave strain. We perform a general
analysis of such possibility within DHOST theories. We use disformal
transformations to find the theory space with larger resonances, within an
effective field theory approach. We then apply our formalism to a non-minimally
coupled ultra-light dark matter scalar field, assuming the presence of a
primordial gravitational wave background, e.g., from inflation. We find that
the resonant peaks in the spectral density may be detectable by forthcoming
detectors such as LISA, Taiji, Einstein Telescope and Cosmic Explorer.
