I thought it was the plastic pillar molding in between the rear glass and the rear door. Im not exactly sure whats making the sound technically, but I can recreate it by pusing on the upper corner of the rear glass sitting in the backseat with one finger and quickly letting go. It is a clicking noise that drives me crazy when I go over bumps or uneven roads. Anyone else have this noise?

 

I have this exact same problem in '02 TL-S. Let me know if you can solve it--I'm going on a 1200 mile trip to Florida and don't want to hear that all the way there!