I have a 1986 GL1200 Interstate. It has developed a knocking sound whenever I accelerate or decelerate, kind of like someone hitting the area around the transmission with a hammer. It is only one knock each time. There is no vibration, just a single knock each time I give it throttle, or release the throttle. I was thinking it was the Universal joint, but I am not sure. Any ideas on what is causing this sound?:?