Pneumatic noise in 737 cockpit below 4000'
Under each cockpit sliding window there is a small drain hole the thickness of a pencil. It is the drain from condensation formed behind the overhead panel. You will see the clear plastic tubes leading in the corner of the sliding windows. This drain hole opens and closes by diff pressure (ball on a spring). I believe that's the noise you hear and not the overboard exhaust valve several meters behind you and under you.