You're saying the sound is coming form the outside? If so, I don't know what could make a squeek, i thought most things went knock, bump or scrape when outside, I'll leave that up to the others for suggestions.
Don't forget the spare tyre will squeek quite happily if it can get to the body work. On my 145 the annoying squeek from the back turned out to be the catch on the boot being loose.
Maybe you should get someone to drive or sit in the back while you drive and try to locate the source. Or, take everything out of the boot and tighten any screw/bolt you can see.
Good luck, I know how absolutely annoying squeeks can be!