there's various techniques to stopping audible rattles and resonance, and it boils down to:
1. reinforcing
2. adding weight
it's important to locate exactly where the rattle is.
to reinforce, you can either tighten any existing bolts, or add more, or use something like expander foam, sprayed (SPARINGLY at first) into the area between the rattling panels. you could also wedge/sandwich some foam rubber between 2 rattling panels if this is the problem.
adding weight reduces the resonance frequency of the panel. essentially, it will resonate at a lower frequency, which is likely to be inaudible. you use self-adhesive sound deadening matting, of which there's many brands/series out there. some of the best includes Dynamat Xtreme and Stinger Roadkill Expert. both are marketed as car audio sound deadener.
apply it to the rattling panel/trim, and it should improve.
i presume you're prepared and know how to dismantle the area, otherwise you may not be able to tackle it properly. rear seats need to come out to remove the parcel tray. from underneath, the boot liner comes off fairly easily with its twistlock tabs and clips.
