Over a period of months, maybe a year when I drive over dips or bumps at speed, I hear a LOUD BANGbang sound when the rear wheels hit the bump. The noise comes from the auto-shift box to the right of the stick. When I place my hand in that area, I can feel the percussion too. On the highway it is so loud it sounds like a .22 discharge and always startles my passenger, or me. It only occurs if I hit a bump around 40 or over, and the amplitude of the sound is proportional to speed. At 40 it sounds more like a CLICKclick, and I can feel that too on the side of the gearbox. please tell me what it could be, and should I be concerned about safety or an exspensive repair? Thanks for your wisdom.