I know you said hissing was comming from plenum, however i had exact symptoms and slight hissing sound, it was the turbo to front pipe gasket had blown a chunk out. try finding the hiss with a silicon pipe/tube in your ear. worked for me.
Hey ive looked around and ive got the cash so can you send me your account details please.
Also needs to be shipped to Cairns QLD id PM you but aparently i have to have 10 posts before i can do that
I txt'd you earlier regarding this
cheers