Sounds like the overboost is kicking in, the signal will come from the ecu I suspect.Alternatively maybe the boost level is set to high and may need to adjust it on the actuator to drop the boost a touch.
You need to know how much boost it is supposed to be running firstly, secondly check it on the boost guage,I'm assuming you have one on the car.You could have an air leak and hencethe turbo could be overspeeding possibly leading to overboost.

Finally I would get the car set up by a specialist to check boost and AFR levels etc especially if you have just purchased the car in your case.A good set up by a specialist is worth it's weight in gold.
I hope this helps.

Thanks. Lee.