i had a similar noise with my s2 recently,had intercoler pressure checked,changed ex manifold gasket,throttle housing gasket and trying different boost hoses i was at a loss until i spoke to universal turbos and they said if when you come on boost the noise appears take your foot off the accelerator and dip the clutch,if the noise goes straight away then its a faulty turbo,if the noise decreases with engine revs then its an air leak.mine was the turbo (an inbalance in the shaft).hope this is of some help....