mate,
i had this exact problem.. i too hav my turbosmart plumback bov coming off the main fmic piping on my gc8..
went to welshpool to get the car registered (bought it from victoria)
the solution is incredibly easy and i discovered it by accident when trying to adjust my plumback to make it quieter at the pits..

i unscrewed the "loose" and "harden" twist top too much and the top popped off..
took the spring out of the bov, put the top back on it and connected again to the piping.. turned on the engine.. revved it..

dead quiet..

while its not sumthin i recommend u doing for long periods of time, ultimately it satisfied this guy that there was no noise, even when he drove it around the block.. u couldnt hear it..

car registered