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.