Tgo

Hello, I am wondering if one of you can help me diagnose a loud hissing noise from my e60 520d lci. The noise is present when the car is idling and is comming from near the turbo, and can be heard clearly from the bottom of the offside wing, near the wheel. It is also accompanied by an intermittent screeching noise when the turbo kicks in. Also it may just be me but the car also feels as if it is not as responsive as usual. I looked into it the other day but the only thing I could find was some oil seepage from the charge pipe to the intercooler.
any ideas are greatly appreciated.
Many Thanks