Clunking sound after revving engine

Hi I have a 95 vulcan en500A and recently while riding I revved up through a couple of gears and then slowed down at a stop light. As the bike revved down I hear a clunking noise that sounded like it came from the head of the engine. When I got home I tried to get the bike to do it again and I couldn't get it to. Since then it has done it on occasion. It seems like it only does it when I decelerate and on the down side of a Rev. Any clue what it could be?