I have the same problem, still not sure what it is though. I also have a problem with drivng the car as well but I'm not sure if the 2 problems are connected. When I boot it the car coughs a bit through the top end of the gear when the turbo is in. If I'm in a higher gear where it takes longer to get through the gear I get a bigger cough, a bit like a bang. Anyone got any idea what this might be, I'm hoping it's a problem with the one of the injectors.