I have a late 2002 996 Tiptronic Turbo had it 4 years its never missed a beat. Out in it the other day car ran fine then later on in the journey I floored the throttle at around 20mph and there was a huge bang from the back! There was a continual grinding knocking noise with each wheel revolution as I slowed down it drove into a car park I was right next to but once the RAC came it lost drive. The engine has good oil pressure and ticks over OK but it sounds to me like the box has gone. Anyone any ideas what it sounds like and if its a box what sort of money am I looking at?