Last one of these I had with similar problems had a leak in the intercooler. I found that you can block the induction pipes with carefully selected aerosol caps! Took pipe to intercooler off turbo and blocked it, took pipe to manifold off and blocked it with a cap with a union fitted in it, blew in air from a pressure regulator at a couple of p.s.i and then had a listen! Bit fiddly to do but it used gear lying around the workshop, no special equipment. HTH Shaun