  We study the effect of an ultra-light primordial black hole (PBH) dominated
phase on the gravitational wave (GW) spectrum generated by a cosmic string (CS)
network formed as a result of a high-scale $U(1)$ symmetry breaking. A
PBH-dominated phase leads to tilts in the spectrum via entropy dilution and
generates a new GW spectrum from PBH density fluctuations, detectable at
ongoing and planned near-future GW detectors. The combined spectrum has a
unique shape with a plateau, a sharp tilted peak over the plateau, and a
characteristic fall-off, which can be distinguished from the one generated in
the combination of CS and any other matter domination or new exotic physics. We
discuss how ongoing and planned future experiments can probe such a unique
spectrum for different values of $U(1)$ breaking scale and PBH parameters such
as initial mass and energy fraction.
