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Interesting, I have the same issue. Similar sound. Only difference is that I have an aluminum inlet tube. But sometimes when I blip the throttle, it would make that noise. I know when I would cold start my car on e85, it would make that similar noise also. I data logged on start up one time and it didn't detect knock. I think my issue is different as yours seems to be strictly with air pressure.
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Vortech SC on 9psi Pulley, Invidia Catless Front Pipe, Invidia N1 Catback Exhaust 
242whp/197ft-lb on Mustang Dyno (91 octane water)
284whp/223ft-lb on Mustang Dyno (E85)
Delicious Tuning |