We develop a reduced frequency-response model for how a stably stratified layer beneath the core–mantle boundary reacts to low-frequency motion in Earth’s fluid core. In rapidly rotating magnetohydrodynamics, disturbances separate into a fast inertial branch and a slow magnetostrophic branch; vertical confinement of the slow branch yields a discrete set of trapped magnetic-Archimedean-Coriolis (MAC) modes. Each mode is driven in proportion to its spatial overlap with the incoming disturbance. Near resonance the disturbance transfers energy efficiently, and for steady forcing the lid responds with a Lorentzian amplitude whose width is set by detuning and damping. For core-like parameters the gravest mode has a decadal period; the response amplitude, however, depends sensitively on the assumed forcing profile and on effective magnetic damping. Numerical simulations confirm the predicted fundamental period and show that forcing tuned to this frequency excites the lid far more strongly than detuned forcing. A stratified lid can therefore act as an active spectral filter rather than a passive boundary.