Guys. This topic has been discussed at great length at www.CaymanClub.net. There are dozens of threads discussing it over there.
It appears to be a common and still unsolved problem affecting the Cayman. The clunk is caused by a resonant counter weight that is installed in the hatch itself. Because the hatch is quite heavy and it sits on a flexible rubber seal, when the car goes over an abrubt bump the rubber seal will compress and the hatch will vibrate. Because it has a lot of surface area, this sends a loud audible bass note through the car. The weight changes the resonant frequen