As electrification trends for the automotive industry have accelerated and the demand for high efficiency has increased, hybrid transmissions have been pushed to enlarge the operating range of EOP. And in conventional transmissions, an Idle Stop and Go (ISG) EOP is becoming imperative. The operating conditions of the ISG EOP make it difficult to gain advantages by masking the effects of engine firing or drive noise. Thus, it is necessary to study noise in operating ISG EOPs. Also, the EOP inner components require precise processing and manufacturing because they should be made to a compact size owing to vehicle layout limitations. This paper first describes the results of an experimental study on the abnormal vibration phenomenon, which makes EOP operating noise worse. And secondly it was investigated the cause of abnormal vibration phenomenon which occurs due to processing and manufacturing problems in the process of developing the operating noise of the EOP for ISG on FF type automatic transmission. Finally, the verification results after improvement were described.