  We propose that there may be a substantial stochastic gravitational wave
background from particle origin, mainly from the gravitational three-body decay
of the inflaton. The emitted gravitons could constitute a sizable contribution
to dark radiation if the mass of inflaton is close to the Planck scale, which
can be probed by future CMB experiments that have a sensitivity on the
deviation of the effective number of neutrinos in the standard cosmology,
$\delta N_{\textrm{eff}}\sim 0.02 - 0.03$. We have also illustrated the
spectrum of the radiated gravitational waves, in comparison to the current and
future experiments, and found that gravitational waves from particle origin
could be the dominant contribution to the energy density at high-frequency
domain, but beyond the sensitivity regions of various experiments.
