Two-phase flow in ducts, especially in intermittent patterns, presents unique challenges due to the complex interaction between the liquid and gas phases. This article investigates fluid–structure coupling in horizontal ducts transporting gas–liquid two-phase flow, focusing on the amplification of duct vibration at the cut-on frequencies of circumferential wave modes. This amplification, resulting from the interaction between fluid pressure waves and the duct’s structural modes, is particularly significant at specific frequencies where energy transfer occurs between the fluid and the structure. Using acceleration, pressure, and void fraction measurements, the study demonstrates that the interaction between fluid pressure waves and the duct’s structural modes leads to a significant increase in vibration at specific frequencies. This amplification occurs near the cut-on frequencies of circumferential modes, where energy transfer occurs between the fluid and the structure. The research explores the localized nature of this coupling, utilizing the coherence function to show that the coupling is stronger in specific regions of the duct. Additionally, the time-domain modulation of the vibrational response caused by the passage of bubbles and liquid pistons is investigated. The results demonstrate that the bubble passage frequency can be estimated from the demodulation of the acceleration signal filtered at the cut-on frequencies. This finding suggests that the duct’s vibrational response can be used as a non-intrusive indicator of the void fraction in two-phase flows.