This paper aims to summarize the study on the radiated noise effects of plastic intake manifold on a diesel powered inline 5 cylinder engine. At the inline 5 cylinder diesel powered engine development program excessive noise has been measured at nearly 800Hz during the Pass by Noise homologation tests. This engine would be installed in to light commercial vehicle so the pass by noise level must be under 75db(A). The engine is 3,2 liter turbocharged diesel engine which gives 200 PS as an output. As customer satisfaction is an important parameter, interior noise level was also measured and found to be over the limit. To identify the root cause of the problem, engine has been installed into NVH (Noise, Vibration, and Harshness) test room. Acoustical mapping technique was used throughout the engine to find exact location of noise source. As a result of the test, intake manifold has been discovered to be problematic with respect to NVH properties. The problem has been identified at the late stage of program because the first vehicles have been tested with aluminum intake manifold as the tooling time of plastic intake manifold took about a year. During the design of intake manifold, material has been chosen as a plastic instead of aluminum to decrease the material cost and the total weight of the engine. In order to determine problematic locations over the intake manifold, transducers installed various locations on the plastic intake manifold in a semi anechoic engine dynamometer. For the cost and timing advantages, FEA tools have been used to solve the problem. Initially, structural modal analysis has been conducted on the manifold to determine the mode shape and resonant frequency that may be within the part. This information was used to compare and ideally correlate the frequencies that are highlighted in the test data that previously obtained. Then, point mobility analysis has been run to determine the position and magnitude of the movement within the manifold structure, this enabled to pin-point where stiffening is required. 3-dimensional engine vibration data (hz vs G amplitude) has been used for this analysis. Once the point mobility study was completed, the required model changes have been implemented to stiffen the component in the required areas. Intake manifold internal design could not be changed because of tight timing plan so just external design of the intake manifold has been revised. Many analyses have been run with different wall thicknesses and different rib locations to reach the best optimized NVH performance that combines weight and cost advantages. Mainly there were 4 different models which have been analyzed; 2,4mm wall thickness without rib, 4mm wall thickness without rib, 2,4mm wall thickness with ribs, 4 mm wall thickness with ribs. After having a comparison table, FEA results have been verified with vehicle, engine bay and pass by noise measurements. As a result of the several iterations wall thickness of the part has been increased to 4mm and several locations of the part have been ribbed. With the help of all these changes listed above, the vehicle has passed the pass by noise homologation test successfully.