vibration can cause problems. Sources are unbalanced props or spinners (especially if plastic or pot metal).
Tank problems can be kinks in the brass tubes or tubing, holes in the fuel lines, pinched fuel lines, etc. One problem is that the clunk inside the fuel line is connected by a short piece of fuel line and that line may grow with exposure to the fuel. So it is possible that the clunk is being blocked at times by the back of the tank. It should be about 1/4" short of touching the back end of the tank when it is made.