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We propose and demonstrate a technique for coupling phonons out of an optomechanical crystal cavity. Using a perturbation that breaks a symmetry in the elastic structure, we induce phonon leakage without affecting the optical properties. It is shown experimentally via cryogenic measurements that the proposed cavity perturbation causes a loss of phonons into mechanical waves on the surface of silicon while leaving photon lifetimes unaffected. This result demonstrates that phonon leakage can be engineered in on-chip optomechanical systems. We experimentally observe large fluctuations in leakage rates that we attribute to fabrication disorder and verify this finding using simulations. Our technique opens the way to engineering more complex on-chip phonon networks utilizing guided mechanical waves to connect quantum systems.