Hello,

My father recently purchased a 2000 Grand Marquis. Tonight, he called and was asking me what I though about a strange "bump" sound from the rear of the vehicle after startup. He says that it doesn't happen every time, but in the mornings, when he goes to lunch, and on the way home, right after he starts the vehicle, he'll get this noise. He says it sounds just like someone walking up behind the car and smacking the trunk lid.

He theorized that the fuel pump was drawing a vacuum, and that maybe the gas cap wasn't venting properly, thus partially collapsing the tank. I shook my head at this, though I have no idea if his car has a fuel return on the rail, or if that even matters. (it's a 4.6) I speculated that since the car has air-ride, maybe there's a bushing or something failing, and the charging of the suspension system after startup is shifting a geometry somewhere, and popping.

What would you guys think? Thanks for reading my first post here.