Abstract: Power transformers are core components of power systems, and their operational reliability is essential to system security and stability. This paper analyzes a case involving abnormal noise from a 110 kV transformer during operation. Dissolved gas analysis initially indicates high-energy arcing inside the transformer. Further measurements of moisture in the insulating oil and partial discharge tests support the diagnosis of moisture ingress into the internal insulation and a discharge defect. Equipment disassembly reveals distinct ablation sites in the interturn insulation at the oil baffles of the phase A and phase C high-voltage windings, confirming the nature of the fault. This case demonstrates the limitations of conventional tests in diagnosing certain insulation defects and provides a practical reference for diagnosing similar latent transformer faults.