have you considered removing the boot shelf and putting it back again?
one of my window seals kriiik kriiiked or atleast made a similar sound until ive had it replaced,try inspecting the seals of the boot/back half of the car.
(the seal creaks happened on bumpy roads or when escalating hills and going down slopes but only at the beginning.....they did not make any sounds when braking though....ofc expect when i was breaking on bumpy roads then yes they did make a sound)