If it does it when you use the motor to slow down (pull lots of vac) then it may be your valve stem seals. If it does it at idle also it could be your turbo seals. Do you have a restrictor in the turbo. Also, you can pull the down pipe to see if it is wet inside, then pull the manifold and see if the exhaust runners are wet. This will help tell if it is turbo or motor.