Air sound when shifting early?

I just want you all to do a simple task to me while you're driving in your stang. The task is to drive normally but when you get to 30-40 mph, shift to 5th gear and press on the gas pedal and see if there's hissing/air sound present (with windows up). I'm asking you all this because I want to make sure if that's it's normal. The sound seems to be coming from the very back of the car. Peharps it could be exhaust leak.. but please test it to confirm if it's normal or not.. thanks
 
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Well I did some searches and found out that it could be the high flow cats sound.. when shifting at high rpm (2.5+) there are no hissing sound what-so-ever. The exhaust mod that I just installed on the car a month ago are

BBK LT, BBK x w/cats, Magnapack

I encountered a header leak from driver side. I replaced the gasket and the ticking noise was gone. Now it's the hissing noise coming from mid-rear part of the car. Any ideas?
 
whenever you add high flow components to your exhaust you will hear some type of hissing. this is just the air flowing

i have a hissing in 5th gear as well but mine is due to the fact i have an aftermarket shifter which allows for more noise.

i remember that when my dad put headers, high flow cats, and x pipe on his vette it hissed under accel because of the increased airflow.