That was Tom and Ray's guess. But, well, they were the first to admit that the answer might be a clunker, in more than one sense of the word.
What do you think? Any suggestions? Thanks-- and we'll ask Wendy to drop by right here.
I cannot explain the short drive making the noise louder but in this model the fuel pump is in the gas tank. The pump could have come loose from the mounts and banging the sides of the fuel tank. Where she explained noise coming from would be correct.
I think this could be the vent valve (shutoff valve) for the fuel tank sticking shut, or closing when ignition on, due to a mechanical or electrical fault. If the valve closes, and the purge valve opens, this could cause the fuel tank to contract. If it is an electrical fault (shorted signal wire to vent valve) it would be possible when power(ignition) is off that the valve would then return to its normally open state.
Vehicles with metal fuel tanks can undergo a phenomenon called “oil canning” if high vacuum builds up in the vapor space above the liquid fuel. Vacuum within the tank allows atmospheric pressure to deform the sides and bottom of the tank inward. A distinct metallic “clunk” results when the surfaces snap back into their original position when the vacuum is suddenly relieved. A plugged atmospheric vent in the carbon canister of evaporative emissions system might create such a condition. Also, if the purge solenoid that meters fuel vapor from the fuel tank into the engine is stuck in the open position, vacuum would begin to form in the fuel tank as soon as the engine is started, possibly explaining the symptom during short trips. The fuel cap itself acts as a safety valve, designed to vent the tank to atmosphere before the tank is damaged due to either an over-pressure or over-vacuum condition.
To determine if the the evap system is the “clunk culprit,” listen for a hissing noise at the fuel filler as the gas cap is slowly removed. Operate the vehicle with the gas cap removed for a few days, which will maintain atmosphereic pressure inside the fuel tank. If the clunk is still audible, then the evap system is not the culprit. If the clunk goes away, some further investigation is needed to isolate whatever component(s) in the evap system are defective.
i had exactly this same problem with my 2001 saab 9-3 hatchback and thought that the sound was the antenna. it was diagnosed as a loose bolt holding the heat shield in place, and turned out to be a simple and fast repair once up on the lift. all best.
I think it might be her oil pan flexing when the oil flows back into the pan. I bet it makes a similar noise, at a higher pitch, just after starting, but isn’t noticeable from within the car. The noise she is hearing would be louder on a short drive because everything is still relatively cold and hasn’t expanded. Expansion would reduce the noise. There are 3 large faces of metal on the oil pan and each would have a different sound. The sound wound be a deeper tone because of the amount of oil in the pan. The oil doesn’t flow back into the pan the same way every time so it makes a different rhythm each time she stops.