  We show that primordial black holes - in the observationally allowed mass
window with $f_{\rm pbh}=1$ - formed from late nucleating patches in a first
order phase transition imply upcoming gravitational wave interferometers will
see a large stochastic background arising from the bubble collisions. As an
example, we use a classically scale invariant $B-L$ model, in which the right
handed neutrinos explain the neutrino masses and leptogenesis, and the dark
matter consists of primordial black holes. The conclusion regarding the
gravitational waves is, however, expected to hold model independently for black
holes coming from such late nucleating patches.
