I have a rattle that I need to chase down. Cold weather and no garage have prevented me from doing much. It sounds like a thin piece of metal that is loose and flapping in the wind when I'm driving. From the front it sounds like it's in the left rear area. I've noticed what I though was the gas cap rattling when I shut the rear door, but that's not it. Last night while filling up I opened and shut the rear door and the rattle was lower than the cap. Any thoughts as to what to look for?

 

Kalahari, there are some vents under the rear windshield that are visible when the trunk lid is open. Inside those slots are flaps that open and close, I think they are used for some form of ventilation involving the trunk and inner cabin which is why the noise is so pronounced from inside the car. When the tApe on those flaps wear out, it leaves the flaps on their own to clank against the metal. I used Popsicle sticks to apply glue to the edges of the flaps to glue them down. Good luck.

 

I don't know if you want to glue them shut, it would be okay to leave them open a bit and fix them in place I suppose. They allow flow-through ventilation; air comes in at the cowl and exits through these vent flaps. If you look up from inside the trunk (looking up from under the rear window shelf), you will see the bottom of a metal tray/drain pan. It catches any water that comes through the vent holes in the rain gutter channel around the trunk. This drain pan can be removed and new padding can be applied to the vent flaps.

This "cowbell" noise may not be the noise you describe, so take a look here for more description of the issue we are talking about:
http://www.jag-lovers.org/xjlovers/xjfaq/cowbells.htm