Aftermarket blow off value for 997TT
you can certainly hear them but it's a recirc style (the correct for running in these cars with MAF's etc...) and not a vent to atmosphere....
http://www.sharkwerks.com/products.php?pid=170
http://www.sharkwerks.com/products.php?pid=170




each part gives a tone and all togeather sounds very good