The creaking noise I had reported earlier was resolved last week by the repair activity noted below. The issue was with the roof bow, not the rear parcel shelf as previously suspected.
My understanding is that sometimes there isn't enough glue applied during manufacturing, and if so, it can cause a plastic-on-plastic creaking/rubbing noise inside the cabin. Since the repair, I have driven the car in various conditions, including after it has been sitting in the sun and during the early morning when it's cold. In both situations, I would previously hear the noise, but none has been present since the fix.
I do think there's a second creaking noise, which I generally only get when I pull out of my driveway (when the rear passenger wheel rolls off the sidewalk while the rear driver's side wheel is on the sidewalk). That tends to only happen in that situation, and the same creak was present in the GT loaner I was given. My best guess is that this issue is related to the rear parcel shelf. I'm fine with that as long as it doesn't continue while I'm driving.
Quite pleased to get this resolved.
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