Update. Got the truck back a week ago. No more rattle. Diagnostic Specialist said the mastic near the c-pillar/roof skin failed. They sealed it up and no more noise 
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Can you please show me where they added the tape?We finally have a fix. After 3 separate trips to Newport Lexus, 3 visits to Mission Viejo Lexus and miles of drive time with the shop foreman, they found the rattle and crackling sound in the drivers side door. The door panels on both the front and rear driver side were the culprits. They added felt tape behind each panel to stop them from rubbing on the metal of the door and the sounds are gone. No more B-pillar rattle or loose metal sounds and I couldn't be happier.
Did you get yours figured out? I am in the same boat here and would really like to try to figure this out myself before getting the dealers involved.OMG I just did a Google search to see how to remove the trim from the passenger B-Pillar for the same issue and ended up here. I should have started here to see if this was an issue. Stupid rattle. Now I have something I can show the service advisor when I take it in. It is so irritating and I haven't reported it yet (I only have 10k on GX after 12 months) because it is intermittent and I didn't want to talk with someone at the dealer and say, well it rattles but only sometimes. Now I have some concrete info to give them. Thanks you!
No. If it is what this thread is indicating I don't want to mess with the headliner. It is a pain to remove. I haven't taken it to the dealer yet either.Did you get yours figured out? I am in the same boat here and would really like to try to figure this out myself before getting the dealers involved.