2017 x35i strange whistle in rear by tailgate

[Trehutch]

I have a weird whistling that happens near the passenger side tailgate vent inside. It happens when all doors are closed and the heave is turned on. Open a window it goes away. Close a windows it comes back. I originally though5 it was the blower however not so sure since the sound is isolated in that area. I am thinking something has become stuck in the vent and I need a fish line to dis lodge it. Question where is there access to this in the rear seat. I need to go from front to back when i do this so I don’t make it get stuck more whatever it might be. Hoping easy fix since I was originally going down a blower replacement front and rear

Thanks for any help