Thus, the engine of your car burns the right amount of air/fuel mixture for the smooth combustion process. During acceleration, many times you will hear some metal knocking noise from the Porsche Cayenne that gets louder and faster when you speed up the car. Apparently, the knocking noise can be more noticeable when you are running the engine under a heavy load. Go through the following slide to know about the causes behind your Porsche Cayenne producing a knocking noise when you accelerate.