your clunk could be flipped around to the front of the tank. Take the plane and give it a good shake. Im not kidding, thats all it takes to fix that problem if thats what it is. Before that youshould gently shake the plane to see if you can ear the rattle of the clunk and if you dont then thats what it is. or you could have a pinhole in the fuel line. If there arent any obvious leaks then it could be inside the tank.