  Recently the international pulsar timing array collaboration has announced
the first strong evidence for an isotropic gravitational wave background (GWB).
We propose that rapid small oscillations (wiggles) in the Hubble parameter
would trigger a resonance with the propagating gravitational waves, leaving
novel signature in the GWB spectrum in the form of sharp resonance peaks. The
proposed signal can appear at all frequency ranges and is common to continuous
spectrum GWBs with arbitrary origin. Due to its resonant nature, the signal
strength differs by a perturbation order depending on whether the GWB is
primordial or not, which makes it a smoking gun for the primordial origin of
the observed GWB. We show that a large part of the parameter space of such
signal can be constrained by near future PTA observations, while fitting the
signal template to the current NanoGrav 15yr data already hints an interesting
feature near 15 nHz.
