Knocking Sound from trunk area

[adhx]

I have a 1996 Oldsmobile 88 LS with a 3.8L V-6 engine.

I hear a knocking sound from the rear of my car. The sound is like a single muffled knock. It's like someone is inside the trunk and gives it a single knock with a rubber hammer or fist. You have to listen for it sometimes, but you can hear it.

The sound usually occurs when I first drive the car in the day, after I have parked the car for a while. It only happens one or twice usually within a few minutes after starting, and then I don't hear anything more. And it can happen either while driving or while idling at a stop.

I checked my trunk and there is nothing loose. I thought it might be the rear shocks or brakes, but I had these checked out okay.

Someone told me that the sound could be coming from the gas tank, with the problem either being:

1). The gas tank is pressurizing from the fuel, or it might be running a leak check and pulls a vacuum. This could cause the gas tank metal to move in or out, giving a dull thud sound.
2). The baffles inside the gas tank have come loose and this is causing the sound somehow. One symptom of this baffle problem is that you can hear the fuel sloshing around in the gas tank. And I do hear my gas sloshing sometimes around when I back up.

Any opinions? Is this something that I need to fix, or can I just leave it be?

[Blue Bowtie]

Maybe it's time you let the ex-governor out of your trunk.

Since the shocks were checked and the upper mounts were given a thumbs-up that eliminates my first guess. Does the vehicle have a ride-level system?

[D Ryan]

tire ware can cause this. look for cupping (tire ware)! maybe rotate tires!

[inafogg]

are we sure its not the pump??or it doesnt have auto leveling??
guts thinking.....

[adhx]

What are a "ride level" system and "auto leveling"? And how would they possibly cause this knocking sound? I don't believe I have those systems.

I had the tires rotated last year, and the sound doesn't seem to be coming from the tires.

[D Ryan]

Originally Posted by adhx

What are a "ride level" system and "auto leveling"? And how would they possibly cause this knocking sound? I don't believe I have those systems.

I had the tires rotated last year, and the sound doesn't seem to be coming from the tires.

Reason I said this is I borrowed my moms car to take my brother to the airport, on the way there I hear thumping in the trunk area, at certain speeds it was loud, I had to pull over and look under the car to see if something was coming apart. This was on a 04 Buick. Mom never gets the tires rotated,and never drives over 55mph they were cupped and that was causing it to make the sound, changed the tire and all was well.