Thanks for your post!!!
I was on the verge of getting rid of my car as the creaking noise from rear vent upon temperature change was driving me insane.
I was getting very severe creaking noise from the rear shelf area as my car was warming up after sitting in a cold parking lot for a while.
The noise was severe enough to make me seek a new vehicle not made by BMW.
With the help of your post, I took apart the rear shelf area. Here is a picture of the plastic vent piece.
I noticed that the plastic vent piece was rubbing against the metal frame of the car!!! You can see the where the paint was rubbed off.
I have applied felt tape where plastic vent touches the bare metal.
I think the problem was that the metal chasis and the plastic vent piece expand and contract at different rates causing the noise.
I guess I'll keep the car for a little longer now.
Before anyone take apart their car, simply push down on the plastic vent piece right by the rear windshield and see if it squeaks or not. Mine was sqeaking like crazy before. Now, it is dead slient.