Flapping And Banging From Rear!

I have had my Mondeo for a few months now (03 reg) and I do love it but a couple of weeks ago it started banging and flapping from the rear. It happens at higher speeds, mostly at 80mph and gets worse the higher the speed, but sometimes at 60. It doesn't happen when going over bumps and is a very random sound. It has been investigated both by myself and my bloke, and twice by the local mechanics and it is still doing it! This may sound a little weird but it actually sounds like there is someone in the back trying to get out and they are banging something - it gets very loud! The local mechanic said it was a flap under the wheel arch to the rear of the wheel and fixed that but it still does it. Someone else suggested a bush - but mechanics have tried (I am assuming) that avenue.

I would be so grateful if someone could shed light on this, before I get the priest in to perform an exorcism on it and get rid of the ghost!!

Andrea