In a spin‐wave instability experiment, a YIG disk was bonded onto a quartz rod with a transducer attached to it, and was placed in a microwave cavity in a parallel‐pumping arrangement. For microwave power above the threshold for spin‐wave instability, parametric generation of acoustic waves was observed. The generation was evidenced by the simultaneous appearance of a low‐frequency oscillation in the reflected microwave power and at the transducer. In a subsequent experiment, a low‐frequency acoustic wave was sent into YIG through the transducer. A great enhancement of the oscillation in the reflected microwave power was observed. The enhancement was due to phase‐locking of spin‐wave modes.