I had the same rattle. It took me 2 days to find the fix.
I had a pair of clip on sunglasses in the case on the roof. Removed sunglasses.........no more rattle.
Then I got a similar rattle a couple of months later. Sunglasses case was empty. This one took a day to fix.
The drivers side sunvisor has been unhooked to cover the side window. When it was returned to its original position, I forgot to hook it into the slot. Hooked it in...........no more rattle.
Hopefully your fix will be just as simple.
2019 Terrain SLE 1.5 with Nav, Safety alert & power tailgate