This hollow tapping/knocking noise is subtle and others claim not to hear it BUT I DO and it is driving me crazy. The sound becomes noticeable at about 50kph and continues to 65/75kph by which time it becomes swamped by normal tire/road/wind noise. The frequency is too rapid for the wheels/tires/brakes but seems to agree exactly with prop shaft rotation. It is present whether accelerating or coasting or even when coasting in neutral. It is clearly not engine related and certainly consistent with some rotating part, not a rattle. I have complained about it to the dealership service department repeatedly for more than a year. Technicians have ridden with me and driven the car but say they cannot hear it, however, after my latest complaint they took my word for it and booked the car in for a prop shaft replacement at no charge. BUT THE NOISE IS STILL THERE! I could hear it within a few hundred metres of leaving the shop. I hesitate to accuse them of going through the motions to try to satisfy me but ... What should I do now?