  We investigate the impact of stochastic quantum noise due to trans--Planckian
effects on the primordial power spectrum for gravity waves during inflation.
Given an energy scale Lambda, expected to be close to the Planck scale m_Pl and
larger than the Hubble scale H, this noise is described in terms of a source
term in the evolution equation for comoving modes k which changes its amplitude
growth from early times as long as the mode physical wavelength is smaller than
Lambda^-1. We model the source term as due to a gas of black holes in the
trans--Planckian regime and the corresponding Hawking radiation. In fact, for
energy scales larger than, or of the order of Lambda, it is expected that
trapped surfaces may form due to large energy densities. At later times the
evolution then follows the standard sourceless evolution. We find that this
mechanism still leads to a scale-invariant power spectrum of tensor
perturbations, with an amplitude that depends upon the ratio Lambda/m_Pl.
