  We propose to use the resonant enhancement of the parametric instability in
an optomechanical system of two optical modes coupled to a mechanical
oscillator to prepare mechanical states with sub-Poissonian phonon statistics.
Strong single photon coupling is not required. The requirements regarding
sideband resolution, circulating cavity power and environmental temperature are
in reach with state of the art parameters of optomechanical crystals. Phonon
antibunching can be verfied in a Hanburry-Brown-Twiss measurement on the output
field of the optomechanical cavity.
