My base 99 (4 cyl, 2wd, auto) has a very distinct rattle when I hit sharp speed bumps or potholes... it's done this since I bought it with 48k miles (I'll turn 100k within the next 2-3 weeks). There's a "flap" of metal inside the frame rail right behind the bumper that seems like it could be making the noise... it's hard to tell since I can't recreate the sound unless I'm driving (i.e. can't bang with hammer to make the same sound)...

Does anyone know what that "flap" is for? There's another flap in the pass. side frame rail too, but it doesn't seem to be as loose as the drvr side? I can't really get in there to get a good look w/o dropping at least the hitch, maybe the bumper too.

Any ideas?