We also noticed 2nd row squeak (not rattle) on our Montreal road trip on the way up, but not on the way back. We had our friends in the vehicle while driving around Quebec, so having had weight on the seat and seat position are definitely contributory. I suspect this relates to the incredibly complex folding mechanism and where the different bits of upholstery end up each time, so will be difficult to diagnose properly.
Oh, we also confirmed that the 3rd row vents do work quite well, and just require the 3rd row being occupied! That's a relief from a service perspective, but I hope there is an override. When we have dogs we have very large dogs, and they sit on beds in the cargo area. I would like to have the vents working back there even with the 3rd row folded.