A suspected cooing sound was identified in the hybrid powertrain under the idle charging condition. Data collection and analysis confirmed that this abnormal sound was a cooing sound exhibiting engine half-order characteristics. Analysis of the generation mechanism revealed that the cooing sound originated from excitation by high harmonic components of shaft system torsional vibration, which was then amplified through the coupling of hybrid powertrain bending-torsional mode and EGR mode. Consequently, three optimization schemes were put forward, including replacing the flexible flywheel, adding a DHT connection bracket, and adding an EGR bolt support point. These schemes successfully eliminated the abnormal cooing sound. The analysis methodology and countermeasures provide significant reference value for addressing similar NVH problems.