We propose a novel mechanism where a first-order phase transition modulates the decay rate of a massive field. This modulation, even if the scalar field has negligible energy density, subsequently generates an observable stochastic gravitational-wave (GW) background. The stochastic nature of bubble nucleation leads to the asynchrony of phase transitions, generating superhorizon-scale density perturbations through spatial variations in the decay rate $\mathrm{\Gamma }$. These perturbations subsequently source second-order GWs with peak amplitudes governed by the phase transition parameter $\beta /H_{*}$ and decay rate $\mathrm{\Gamma }$. We apply this mechanism in the reheating scenario where the decay rate of the inflaton is modulated by the scalar field that undergoes a first-order phase transition. Numerical calculations reveal that the GW energy spectrum typically reaches ${\mathrm{\Omega }}_{\mathrm{GW}}\sim {10}^{-10}$, demonstrating prospects for detection by space-based interferometers like Laser Interferometer Space Antenna, TianQin, and Taiji. This work establishes a new approach to probe phase transition processes in the early Universe without requiring significant vacuum energy release.

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• Evolution of the Universe
• Gravitational waves
• Inflation
• Quantum cosmology