Something loose in fuel tank area

If I grab my car by a front tire and shake it back and forth laterally, there is a clunk-clunk-clunk noise coming from what I think is the fuel tank area that sounds like something loose sitting in a slight valley. As the car rocks back and forth and comes to rest, it clunks loudly and then quieter, and then stops. Honestly it sounds like somebody left a screwdriver inside the car somewhere and it's rolling back and forth when I shake the car. I've had the entire trunk carpet out and all the spare tire and tool stuff, as well as the battery and battery tray, and there's nothing loose anywhere in the trunk or cowl areas. I've also looked on top of the belly pan on the front, nothing there. So whatever it is, it's in a place where I can't see or get to easily, and all sounds kind of point toward the fuel tank. I'm wondering if there's something semi-loose in there? I know some cars have ball valves so if you flip them over in a crash, fuel won't pour out the filler; not sure if this could be some feature like that. I never hear the noise when I'm driving, I've just heard it when tugging at wheels to check for suspension play. More just a curiosity than an actual problem.

C2 or C4? If it's a C4, you can remove the black plastic cap in front of the battery tray and take a look in and around the fuel tank.

C2

...no chance it's a wheel bearing, right? Do you have a glove box? Is there a bottle of sunscreen or something in there? I nearly lost my mind trying to troubleshoot a "problem" with my wife's car, only to eventually come to find out that there was a bottle of sunscreen under the passenger seat rolling back and forth between the seat mounts.

Nope, it's coming from basically the centerline of the car somewhere as far as I can tell. No wheel bearing movement whatsoever. It's something extremely loose, like it sounds like it's moving around an inch or so. Not a slight little amount of play like a bearing or suspension component. Imagine a golf ball rolling around in a cereal bowl and then gradually coming to rest, that's more what it sounds like. I have looked pretty heavily inside the car also, I don't think it could be anything under the dashboard. Are there secret body cavities somewhere behind the trunk where a careless mechanic could have dropped a screwdriver? Like I said I've looked under the battery tray, with no luck. It's a 2001 C2, so no extravagant luxuries like a glovebox Thanks for your help!

There aren't really that many hidden areas. The sides of the trunk where the headlight wiring harnesses are located is probably the closest thing I can think of to a hidden compartment... Here's a gutted '99 to perhaps give you ideas for where else you might look...

Have you verified that the drain hoses on both sides of the battery are in place and secured? They should go from the drain hole to the wheel well.

I had a clunking noise going over rough road and uneven things. It was my spare. It was loose on the mount. Tightened up, clunking stopped. YMMV

Weird! I know it sounds crazy, but stand in the trunk, place feet apart and start rocking the car back and forth. See if you can locate the noise. Maybe place your hands on various locations to see if you can feel something moving. Good luck!