  We examine the effect of a trans-Planckian phase on the dynamics of
inflationary tensor perturbations. To remedy the fact that this regime is not
fully captured by standard perturbation theory, we introduce an effective
quantum noise source, whose role is regulated by the energy scale $\Lambda$.
The presence of the source modifies the initial conditions for the tensor
modes, leaving a distinct imprint. We study the amplitude and shape of the
gravitational wave bispectrum of the model and compare these with their
counterparts obtained under the assumptions of Bunch-Davies initial conditions
and $\alpha$-vacua states. Depending on the value of the scale $\Lambda$, we
find distinctive signatures associated with both the bispectrum shape and the
non-linear parameter $f_{\rm NL}$.
