Well I have removed a hatch from one of these cars before.
They are held on by two hinges.
I have pics that come can be useful because I sell parts. heh
Is is possible that the bolts are not on tight enough on that hatch.
How about where the end of the hatch latches on to the back of the car--maybe that needs lubing?
Is it possible that someon could drive the car for you while you sit in the back of it and figure out where it is coming from?
That is all if it is indeed from the hatch, and not something else.