Predicting noise is a step that cannot be ignored in the automotive industry during the vehicle design cycle. This is classically achieved through Finite Element and Boundary Element methods. When dealing with exterior problems, Boundary Element Methods is quite efficient but may induce ill-conditioned equations. On the other hand, Finite Element Methods, if easier to handle are not initially adapted to unbounded media. In this paper a hybrid method, which combines the advantages of both techniques is presented. This method, inspired from Substructure Deletion Method, which is well-known in Civil Engineering, consists in dividing a complex unbounded problem into two easier ones to solve finite and infinite problems. Instead of considering a geometrically complex structure, a prismatic bounding volume is first studied using BEM. Then a classical Finite Element computation is performed on the volume left between the box and the structure of interest. Advantage of this technique is that when testing and comparing several geometries contained in such a box, only one Boundary Element calculation is needed. Efficiency of this method is discussed in the present document.