What Charlie said,
Any hard portion of the plane that touches the tank can transfer vibration. A soft foam is more effective than a rigid foam.
Check your lines carefully ESPECIALLY right at the ends of the brass tubes. File the tube end edges smooth and be sure the lines don't tug at all. Also lines that are TOO loose and floppy can be an issue. The tube inside the tank can have the same issues as the outside tank lines and can really drive you crazy, I use the soldered on grips inside the tank to avoid problems.