I've had a squeaking noise off and on in my 2003 sedan for a few years now. It came and went away on it's own 3 years ago. Came back last year, and searching around this site found numerous posts pointing towards loose seat back brace bolts. Indeed mine were loose, and tightening them did stop the noise - temporarily. It came back again a few months later. This time the bolts were tight. More research and I found one mention of a person that had a spot weld that needed to be repaired at the dealership in his rear deck area.

I pulled off the driver rear seat back, and poked around till I found the exact spot weld that had failed. It's above and to the right of the strut, or to the left of the rear air bag unit. It's a fairly thin piece of metal and I bent it slightly, so it wouldn't rub, and no more squeaking now.

I believe that the loss of the support from the loose seat back brace leads to that spot weld becoming stressed and failing as the chassis flexes over time. Figured I'd throw this on here in case anybody else runs into this issue and just tightening the brace doesn't stop the noise.

 

Try placing a rag in between certain spots of the metal-metal contacts to see if it is a rubbing issue...