As we move toward the year 2000, plastic components are finding more and more applications under the hood of automobiles. As materials such as nylon continue to proliferate in the engine compartment, automotive designers must begin to analyze and optimize the noise characteristics associated with the use of these materials.For example, many valve covers, radiator end tanks and air intake manifolds are now constructed of nylon materials. Fueled by the lighter weights and lower costs associated with nylon versus traditional aluminum materials, the industry expects to see more nylon under the hood than ever before. But the same characteristics that make nylon attractive, light weight and low mass, provide some unique challenges in managing noise. The lighter weights and lower densities of nylon do not “mask” sounds as well as heavier, denser materials such as aluminum.Therefore, engine compartment designers must find new ways to address noise in under hood components -- preferably before cutting production tooling. By optimizing noise performance before the tooling investment is made, automotive suppliers can realize significant cost and time savings.This paper will examine key issues in noise under the hood and provide specific data on how to handle these issues. The paper will focus on the use ot predictive acoustic analysis to design components, and will include testing and comparison of aluminum and plastic. Techniques for enhancing noise characteristics also will be discussed.