Ever try and to hammer out a dent and the damn thing kept reflecting back?
Oil-Canned:
There is, or has been, a dent in the sheet metal, and you can push it into the correct position, but it just deflects back to the shape it prefers. So you can push on it, and let go, and it makes a sound like the old oil cans, boinky, boinky. Like the metal lid of a pickle jar (or snapple bottle) after it's been opened and the vacuum is lost.
I've seen a half dozen "good" trunk lids with this problem. Its usually caused when the steel streches a bit too much for too long. If the "oil-canned" area is indented, then some guys will fix it with bondo, but then if it's ever pushed form the other side a bit, the bondo just pops out.. yick.
Sometimes it's hard to see without probing the trunk lid a bit.
It can be fixed by a talented eye and a blow torch by shrinking and stretching the metal in the right direction, but they rarely come out just right in my opinon.
Thespazardman