Powertrain units such as engine and gearbox are important components affecting the noise and vibration performance of a vehicle. This paper presents a unique technique that uses structural shape optimization together with an acoustic transfer vector (ATV) technique to minimize the noise radiated from a gearbox casing. The ATVs relate the normal velocities of the gear case surface to the sound pressure at points in the acoustic field around the structure. The ATVs are calculated once only in an initial step using a boundary element method (BEM). By integrating the pre-calculated ATVs into the structural model as a frequency-dependent constraint relationship, the sound pressure at any position in the acoustic domain can be obtained as a response quantity in the FEM calculation. This makes it possible to use the standard structural optimization tools available in advanced FEM software to minimize the radiated noise. The optimization model can include any structural changes which affect the stiffness and mass properties - and thus the dynamic response - of the structure. For the application presented in this paper, the thickness of the gearbox casing and the dimension of its stiffening ribs are selected as optimization design variables. With the service-condition frequency domain engine excitation, the optimization changes the original shape of the gearbox casing to generate an optimized shape with potentially lower radiated noise.