Ok, here's a big shot in the dark....these cars are fairly air tight, well at least mine is, my ears sometimes pop when I shut the door. You could have a slight air leakage in the hatch lid gasket. Try sitting in the car and closing the door with the window open or slightly cracked. Do you still hear your strange noise ?
If so, make an appointment to see your proctologist !!!

Welcome to the forum BTW !