A new generation mechanism for electromagnetic waves near the fundamental plasma frequency is discussed. It is shown that a travelling Langmuir pump wave can nonlinearly convert into electromagnetic, Langmuir and ion-acoustic daughter waves via a hybrid stimulated modulational instability. The linear and nonlinear temporal dynamics of this modulational process involving coupling to a resonant ion-acoustic wave are studied. The wave energy conservation relations are derived. The roles of frequency mismatch, dissipation and wave dispersion in the temporal evolution of nonlinear coupling of two wave triplets are analysed.