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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.