  We consider a mechanism for producing a significant population of primordial
black holes (PBHs) and an observable stochastic gravitational wave background
(SGWB) within string theory inspired models of inflation. In this framework
where inflaton is identified as a non-compact axion-like field, sub-leading
non-perturbative effects can superimpose steep cliffs connected by smooth
plateaus onto the underlying axion potential. In the presence of coupling to
Abelian gauge fields, the motion of axion on the cliff-like region(s) of its
potential triggers a localized production of one helicity state of gauge fields
due to the temporary fast-roll of axion around such a feature. In this setup,
primordial fluctuations sourced by vector fields exhibit a localized peak in
momentum space corresponding to modes that exit the horizon when the axion
velocity is maximal. As an application of this general mechanism, we present an
example of axion inflation which both matches Planck observations at CMB scales
and generates a population of light PBHs ($M_{\rm PBH} \simeq 10^{-13}
M_{\odot}$) that can account for all dark matter. In this scenario, the
enhanced scalar fluctuations that leads to PBHs also generate an observable
SGWB of induced origin at LISA scales. The amplitude and shape of the resulting
GW signal inherits specific properties (such as non-Gaussianity and its shape)
of its scalar sources that may allow us to distinguish this mechanism from
other inflationary scenarios and astrophysical backgrounds. This GW signal
together with an observation of PBH distribution at the corresponding scales
can thus provide a window to the inflationary dynamics on scales much smaller
than those probed by Cosmic Microwave Background (CMB) and Large Scale
Structure (LSS) Measurements.
