Hissing could definatly be the boost guage,i had one do the same. This only happened when the guage was plumbed into the boost side(before the throttle body), i changed it to after and i cured it,lol. Another weird coincedence was that the fuel smell was also coming from the guage, i dont know if it remnants of off throttle fuelling or the massive ammount of on boost fuelling that gave this prob but again it seemed to go away after relocation of guage tee off. Its worth a try, especially if you cant see any external leaks. I know this sounds irish(no offence to the irish) but when testing the co stationary if you smell fuel(through the exhaust)9 times out of 10 its running lean.