What is the knocking noise at the back?

[mikem]

I have a 2003 X5 that I've had from new. About a year ago it developed a "rattle" at the back of the car.

Try as I might, I cannot find the source of it. Checks so far have included removing everything from the boot, and I do mean EVERYTHING. Spare wheel, CD Changer, NAV DVD, Traffic Master, Bluetooth, Tools, Roller Cover, etc. I have also removed the cover on the tow adapter and checked to see if the exhaust is loose underneath. The knock seems more noticeable on warm days, as if something works loose when it gets warm from the sun. I removed all the trim from the upper and lower tailgate to search for a loose wiring loom, again to no avail. I have learned to live with it but every now and again someone travelling in the car mentions it and it sets me off looking for it.
Recently I had someone sit in the back as I slowly drove around. They said that the "knock" appears to be coming from underneath the car towards the back in the centre. Any ideas would be gratfully explored!