Flow-generated noise has recently received a lot of attention within the process of designing exhaust and intake systems. Flow-generated noise can limit the amount of sound reduction a muffler can perform inside the duct. This is more important in the modern system design where mufflers are compact and the flow speeds become higher in different sections inside the muffler. In this paper, two measurement techniques are used to measure the flow-generated sound power inside a duct element and compared to each other. The first is based on the radiated tailpipe noise as an active one-port element and the second is based on the measurement inside the duct using the active two-port theory. The radiated sound power is measured inside a reverberation room according to ISO 3741. There has been a lot of work done to calculate the flow generated noise from simple duct elements but little has been published on full mufflers. In this paper, a small muffler was studied. The flow-generated noise inside this muffler was measured using the two aforementioned methods and the results were compared. A 1D model based on the two-port theory was built for this muffler and whereas the flow-generation mechanisms were included in the active two-port formulation and calculated using different scaling laws from the literature for different duct elements. The results are also compared to the 3D CFD simulation using commercial software. All the two measurement methods and the two simulation techniques were all compared to each other and the agreement was good.