First off, if the sound is like mine, it's a creak that occurs mainly when the body of the car is twisted (like over uneven pavement). And the sound seems to come from right under the dash.
So I removed the CD caddy and the DSP unit, then pulled out the rack and the carpet protectors. Underneath was the airbag sensor and this treated steel crossplate with two bolts in it.
Exposing this made the sound very loud and noticeable when I tried driving it again. That being the case, the first thing I did was loosened the two bolts and took it for another drive around the block. The sound was gone. I tried tightening both bolts again, and the sound never returned.
Somehow, that did it, and the sound was gone after that.
-David