1972 Volkswagen Beetle Unidentified knocking

I have a 1972 Super Beetle, where recently I have been hearing a light to very loud "knocking" noise coming from the rear of the car. This noise only happens when I am driving and the car is in gear; it does not happen at an idle. When I press the clutch, the noise either stops or decreases substantially. The last couple miles I decided to drive it, a "metal to metal" noise came about one time and then that particular noise stopped. The knocking still persists; although not continuous, it does make this noise enough to have me park the car until I can fix it. thanks for your advise.