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Montana With Thump On Gas Tank

I have a 2006 Pontiac Montana SN6. Last summer my brother took me to his favorite outback place in Arizona via 8 miles of dirt roads. When we were almost back to the pavement I heard a loud, low pitched, (a low “B”) thumping sound. I stopped, thinking some part of the suspension was loose, but couldn’t see anything. I’ve seen 6 mechanics since and still have the thump when we cross a bump. Yesterday a mechanic decided it was the front of the gas tank hitting the floor of the car, and he pushed the tank to show it made the low-pitched thumping sound. Now it is 10 times worse.

It happens only when the driver’s side rear wheel goes over a bump. It didn’t used to be every bump, just the significant ones. Now it includes less significant ones too. I took the chairs out of the back, pulled up the carpet, and laid my ear against the floor board to see if I could pin point it. It seems to be in front of the rear wheel, about even with the back of the door frame, 14 to 16 inches into the car. It does not seem to be a vibration against the floor of the vehicle, but may be something hitting the fuel tank. That is now my conclusion since I can’t see anything loose, and it isn’t hitting the car body. Also, the pitch of the thump would be different against the floor of the vehicle, but the tank makes a hollow low “B” when you hit it.

I suppose I can have a mechanic take the tank off and see if something is loose under there, but wondered if anyone might have an idea.

If the mechanic could move the gas tank, doesn’t it make sense that the gas tank makes the noise when it moves?

Yes, but I don’t think the gas tank is hitting the floor. Only the very end of the tank (front) moves and it doesn’t seem to move enough to make that noise against the floor. I think something must be hitting the tank.

You say “he pushed the tank to show it made the low-pitched thumping sound. Now it is 10 times worse.”

If the mechanic could duplicate the noise by pushing up on the gas tank, then I don’t understand what the question is. Have him fix the loose gas tank. Gas tanks should not move at all when installed in the vehicle.

The gas tank shouldn’t move at all. There are two metal straps that secure the gas tank to the bottom of the vehicle. One at the front of the gas tank and one at the rear. If the front of the gas tank moves then I would remove the front tank strap to see if it was damaged/stretched while you were on those rough roads.

Tester

Thanks for the notes. I will have someone loosen the tank in a couple of days. Would still like to hear if anyone has had this kind of problem.

I’ve never heard of the problem before, but have absolutely no reason to doubt that the diagnosis is correct. As everyone else has said, the problem has been properly diagnosed. The tank shouldn’t move.

Yes, I’ve seen and corrected this type of problem before. Between the gas tank and the floor of the car there is (should be) some sort of padding or cushioning strip, placed so the contact of the tank to the floor will not create friction and wear a hole in the tank. Since this material is soft to absorb vibrations and noise it sometimes deteriorates. Or it could have been misplaced at a previous fuel pump service. Anyway, there’s supposed to be something there.