Hello I will help you with your question,
Ride in the back seat while someone drives and put your head in each corner of the back shelf. Listen for the noise to be coming from the top of the strut mount where it is attached just behind the rear seat. If it is coming from there replace the mount. Try removing everything from the trunk including the spare tire.
Let m eknow what you find and I will help you with how to proceed
If you can find a spring compressor you can replace the mount but this is a dangerous operation as the spring has enough energy to kill or maim you and must be released under controlled conditions
Here are the instructions to replace the mount, look them over and let me know what I can do to help you with this
First try torquing the center nut for the strut it may stop the noise. If not the mount is the next likely culprit, with the seat out you can likely feel the noise with your fingers if it is the strut
Go through the part of removing the seat side bolsters by removing the bolts at the bottom of the side bolster and the side trim from the "C" pillar by gently prying the plastic retainers out with a putty knofe or other stiff thin tool and the top of the strut will be acessable from the area next to the rear shelf
I think it is a good choice to leave anything more to an equipped shop
No, its annoying to hear the rattle/thump but you will do no damage if you put it off. Heres to hoping that when you tighten the center nut the noise will be gone!
Let me know if there is anything I can do for you