If none of the above, lets rule out a solid. If we apply liquid, none on the ground, or in the oil tank, nor did you mention poor gas mileage. Only option left is a gas. Is the gas cap gasket cracked, warped, missing? Bump equals sloshing or splashes under the gas cap gasket. That says vapor up and out of the tank under pressure [slosh]. Inside the tank is 14.7 or atmospheric in pressure. But when the sun beats down on the metal... make sense we narrow down a solid/liquid/gas scenario?None of the above. Also checked the mounts.