I had a thunk whenever I went over bumps, which had been eluding me for a few months. Every time I jacked the car up, the wheel dropped down, and tightened everything up, so I couldn't find the problem. After working on a few other things on the car, I jumped up and down on the rear lip of the trunk (the part hidden by the trunk lid) and heard the clunking sound.
Turns out the left rear shock bolt, on the bottom, was working it's way out and causing slop in turns and the crazy noise over bumps.
Learn from my adventures, check the bottom bolts as well as the top!