Here's some ideas...... DIY-er stuff that can get you started without dumping a boat-load of money. I'm a big DIY-er guy....... to take as far as possible and when I can't make it happen anymore, then I pay someone to do the rest.
Wires...... wires rattle everywhere. Go through the trunk with a billion zip ties and make sure EVERYTHING is zip-tied down ...... EVERYWHERE. Under the rear deck in the trunk sides...... dig up everything. Wiring can even be found behind the pillar panels in some cars. Some guys go as nutz as EVERY 2 inches there's a zip tie and the wiring is wrapped in something like black hockey tape to cut down on any buzzing.
Dollar store has zip ties cheapo.
Back deck....... remove it and put the box in again to test out what is rattling (after you have zip-tied everything down). If it's good, then check out the deck itself and look for loose bits. This is going to be about securing EVERYTHING down super tight.
Bumper and licence plate........ annoying to some.
Let me know how it goes..........