The main sources of ECS noise in front of the back lavatory door in certain commercial aircraft are mixing manifold radiation and air suppliers. The radiation noise is calculated using CFD and acoustic simulation. Considering the sound reduction of cabin ceiling, the contribution of radiation noise is about 35.7 dBA. The air supplier noise was estimated using transfer function method. The transfer function from air supplier to target position was simulated with a real built cabin model. Then the measured air supplier noise and simulated transfer function were combined to predict air supplier noise at target position which is 60 dB. Considering the sound superposition, the sound power level at target position was calculated, which shows acceptable agreement with test data. This research shows that the main ECS noise source at target position is air suppliers, and that contribution of certain noise source to the target position could be predicted accurately using the transfer function method.