Put some foam rubber between the tank and the fuselage, and put some servo mounting grommets between the tank mounting screws and the plates, so there is no metal to metal or metal to wood contact.  Also, a few tiny drops of Armorall in a gallon of fuel will help keep it from foaming, but use only enough to stop the foaming.  Shake the jug after each drop and see how much it foams.  Stop adding Armorall when the foam disappears as soon as you stop shaking the jug.  You can also get an ABC or Ceramic piston/liner from Fox so the motor vibrates less.

I have had better luck mounting my tanks solid than with foam. Maybe I do not get the right amount or something, but my tanks shake worse, and foam the fuel more when mounted with foam. I do use a little Armorall.