The efficiency of a gap-type of deflector for suppressing vehicle sunroof buffeting is studied in this work. Buffeting is an unpleasant low frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Accurate prediction of this phenomenon requires accounting for the bi-directional coupling between the transient shear layer aerodynamics (vortex shedding) and the acoustic response of the cabin. Numerical simulations were performed using a CFD/CAA numerical method based on the Lattice Boltzmann Method (LBM). The well established LBM approach provides the time-dependent solution to the compressible Navier-Stokes equations, and directly captures both turbulent and acoustic pressure fluctuations over a wide range of scales given adequate computational grid resolution. In this study the same gap-type deflector configuration is installed on two different types of vehicles, a SUV and a sedan. For each car two configurations were tested, with deflector and without deflector. It was found experimentally that the deflector on the SUV significantly suppresses the buffeting phenomenon while the deflector system on the sedan showed almost no impact over a significant range of velocities. Within a detailed analysis process it is shown that the deflector system on the SUV is able to detune the resonance by reducing the excitation efficiency of the shear layer. Based on the analysis a design variation for the sedan was successfully proposed to improve the deflector efficiency for this configuration as well.