hissing? definately sounds like the turbine.
however, are you sure it was a hissing sound? was it very loud? like chaaaaarrrrrr??? coz then u might haf been hearing the wastegate in action.
i tell u, ur turbo car install a wastegate sounds ganas to the extreme man (if you mods allow it la). saw this fierce orange wiralution III blasting down jalan damansara a while ago...sounded like a very loud powerful lorry passing me!!!
~