After examining the rear suspension & exhaust, disassembling the interior of the Cobra clone and the trunk liners to find a rattle in the back of the car, I discover it's....the trunk lid. With the trunk lid latched, I can lift it up & down a little bit and hear it "clunk". Explains why the rattle vanished on the freeway, the wind pushed the trunk down.
I've checked all around the lid and the latch on the lid & the body, and everything is really tight - no movement at all. I can only conclude that the latch itself has a bit of play in it. It's also possible the springs on the corners of the lid are old and aren't holding the trunk lid in place. At this moment, the trunk lid is properly level with the quarter panels on either side, so I'm not sure that adjusting the latch will do any good. Anyone else have this kind of issue? Do I need a new latch, or just new springs on the corners?
pull up on the trunk if its not level with the 1/4 panels id say its the latch. If it is, give the springs a tug and stretch them out. I think there might be some room to move the bottom hook around to get it to latch better as well.
OK, I can officially say my troubleshooting skills need a refresher. The trunk lid was not causing my rattle, it was a loose suspension part. However, I couldn't figure it out until I stuck my head behind the rear wheels and had my buddy push the car up and down. The bracket that holds the bottom of the rear shock to the body had come loose, and was clunking every time the suspension flexed. I had already checked under the car for anything loose, but the bracket only moved when the suspension flexed. When the car's up on jack stands, the bracket looked and felt solid.
Normally, the bolt & nut that holds the bracket are really tight. However, I had to remove and replace them when I installed Mega-Byte Jr lower control arms on the car. The Jr's have a special bracket to adjust the pinion angle of the axle. To better support it, they require you replace the bolt on the shock bracket with a longer one; one that goes through both the shock and Jr's brackets. That bolt was loose, letting the shock bracket rattle around. I had only tightened hand-tight before; now I tightened both sides to torque specs and no more rattle. From my drive earlier today, this one fix solved several noises I've been trying to trace in the Cobra. I need a bit more experimentation, but I might have solved another one.