[Formulabruce]

Mark Donehue's SCCA team really had some experiments with this in 67, 8, 9
The cowl induction builds pressure, but not at slow speeds ( just like your floor vent)
faster is more pressure as the wind makes a small down turn at base of windshield and the rest goes over the car.
This is why the Yenko style hood was developed. ( not sure if they ever made finctional ones though)
Chevy, and chevy powered ponchos in the 60's ( F bodys with 302 Z motors) also grabbed that air from the cowl itself above the heater box with an elbow assembly to the aircleaner to reduce hood drag, and used a flat hood.
The noise is normal, however at slow speeds, like taking off there may be a vacume created at that location.
Wish I had the early camaro wind tunnel pics on disc to show the cowl hood etc working. Very nice work on that air box!! ( nothing beats fresh air in a pontiac!)