  Upcoming LIGO/Virgo/KAGRA (LVK) observing runs are expected to detect a
variety of inspiralling gravitational-wave (GW) events, that come from
black-hole and neutron-star binary mergers. Detection of non-inspiral GW
sources is also anticipated. We report the discovery of a new class of
non-inspiral GW sources - the end states of massive stars - that can produce
the brightest simulated stochastic GW burst signal in LVK bands known to date,
and could be detectable in the LVK run A+. Some dying massive stars launch
bipolar relativistic jets, which inflate a turbulent energetic bubble - cocoon
- inside of the star. We simulate such a system using state-of-the-art 3D
general-relativistic magnetohydrodynamic simulations and show that these
cocoons emit quasi-isotropic GW emission in the LVK band, $\sim 10-100$ Hz,
over a characteristic jet activity timescale, $\sim 10-100$ s. Our
first-principles simulations show that jets exhibit a wobbling behavior, in
which case cocoon-powered GWs might be detected already in LVK run A+, but it
is more likely that these GWs will be detected by the third generation GW
detectors with estimated rate of $ \sim 10 $ events/year. The detection rate
drops to $ \sim 1\% $ of that value if all jets were to feature a traditional
axisymmetric structure instead of a wobble. Accompanied by electromagnetic
emission from the energetic core-collapse supernova and the cocoon, we predict
that collapsars are powerful multi-messenger events.
