I had that noise since I got the car new back in early 04 and still can't get rid of it. I just learned to live with it and just pull down the arm rest when the noise gets too unbearable. The noises do get worst in the winter months. It's been behaving rather well during these southern cali summer months, though. But please do keep us updated when you find a solution since this seems like a very common problem.

My theory is that the 60/40 split of the seatback has no actual upper support where they split (only upper seatback support is on each side of the C-pillar)...thus there's about to be some vibration in that center section of the split area (weakest support point). Being the armrest and the seatback are 2 separate pieces will likely vibrate at different rate...thus causing your noise. I bet if you take your fist and bang on that upper seatback (the 60-split side) just below the headrest with the armrest folded back in the seat, you can replicate you noise. Everything short of strengthening that upper section of the seatback, I'm afraid won't cure your problem.

As an experiment, you can try is to stuff a couple of towels between seatback and the crossbar of the cargo cover...essentially using that cargo cover's crossbar as additional support for the seatback in the middle section...to lessen it's vibration. If that eliminate your noise, then it'll prove my theory.

Hope that'll give you some more ideas to experiment more