Could be the Delta P reaching 2.5 in the descent and the outflow valves and doors opening.
Next time in the flight deck after take off, have a listen to the hiss around you; it stops around 4-5000 feet. Have a glance at the Press page - Delta P should be very close to 2.4-2.5. The valves/doors close and the hiss stops.
The 320 does the same thing in reverse on the way down but at a higher altitude.
Might not be loud enough to hear it from the ground though
