1) Air leak in the tube between the turbocharger and the throttle body mounted on the intake manifold.
2) stuck turbocharger wastegate.
3) cracked, leaking, broken, disconnected vacuum hose on the engine
4) Blockage to the input of the turbocharger (plugged air filter/rodent nest in the air filter box, etc)
5) Turbocharger not fitted properly to the engine area
6) Loose hose clamps on the induction system tubing
7) Cracks in the induction tubing
8) Leaking intake manifold gasket
9) Loose connections at the intercooler (if you have one fitted)
10) Crack in the intercooler (if you have one fitted)