I know that Parcel Shelves are noted as a common cause of squeeks/rattles on the wiki, but I couldn't find any documented 'fix' for it.
Last night when driving home from a meal, I noticed a series of squeeks each time the road was a little rough/bumpy coming from the tailgate. My missus was driving (I'd had a few too many, which kind of explains my next stunt).
I turned the stereo off, undid my seatbelt
and clambered into the back of the car. When putting pressure down on the parcel shelf, you could replicate the squeeking and if you put enough pressure onto the parcel shelf, you could 'compress' it completely and the noise stopped.
I fixed this today by using two packs of 3x adhesive strips from wilko's. (about £1 each). Three strips was just enough to cover the plastic trim 'ledge' which the parcel shelf sits on at either side, so got that lot stuck on - and when stood in the Wilko's carpark I could no longer replicate the squeeks by pushing down on the shelf.
Drove home with no radio on, and the sound is cured.
I can grab some photographs if needed, but wasn't sure if this was worth adding to the Wiki somewhere.