I have a g37s 2011 . HKS intakes, ECU flash , slammed stanced etc etc . Had a full exhaust set up made which is test pipes straight through to 2 powerflow silencers then to tips. When i come off the throttle at low speeds you can hear a short intake hiss from underneath the car (around 4-5 seconds) . All joins are fitted perfect with no leak same for 02 sensor etc. Any ideas what this could be im baffled !!!

 

VQ hiss, it's endemic to the platform and is exacerbated by test pipes. Short of resonating the crap out of your exhaust, nothing will change that.