  We point out the peculiar form of the gravitational wave signal expected from
a gas of particles carry spin 3/2 produced during preheating. Given the very
few ways that gravitinos can manifest themselves in an experimentally
observable way, we stress the importance of improving the sensitivity of
ultra-high frequency detectors in the future. This review is based on work that
appeared in [arXiv:1811.11774 [hep-ph]].
