<p>Tunnel-induced noise amplification has become a major constraint for high-speed trains. This study employs a 1/10 scale three-coach high-speed train model, using the improved delayed detached eddy simulation (IDDES) method coupled with the perturbed convective wave model to investigate the unsteady flow evolution, aerodynamic noise source distribution, and near-field acoustic characteristics of high-speed trains under open-air and tunnel conditions. The results show that the blocking effect of the tunnel wall enhances flow compression, increases local velocity, and aggravates flow disturbances and pressure fluctuations near the pantograph and tail car. In the tunnel, the total sound source energy reaches 1.14 × 10¹² N²/s², 5.26 times higher than in open air, with significant increases in the tail car, bogies, and pantograph. Bogie noise concentrates in the 50 to 1000 Hz range, while pantograph noise dominates from 1500 to 2500 Hz. Tunnel conditions further enhance peak distributions in the low and medium frequency bands. Although pressure disturbances on the train surface are mainly dominated by hydrodynamic effects, the radiated acoustic energy of the sound pressure levels on the roof and side surfaces is amplified by 33.3 and 22.6 times, far exceeding hydrodynamic energy amplification factors of 8.6 and 6.3. The study reveals coupled flow and acoustic mechanisms in tunnels, supporting noise reduction design for high-speed trains.</p>