Page 45 in Chris Brady's excellent "The Boeing 737 Technical Guide", second paragraph down, in the section entitled "Equipment Cooling" sheds some light on this.
Basically, supply-fans and exhaust fans push (and pull) cooling air from the cabin through the flightdeck panels, display units, circuit-breaker panels and E&E bay...and this cooling air has to go somewhere after it has done its job.
In the Classics, this cooling air is dumped overboard by the flow control valve. But in the NGs, it is vented via the overboard exhaust valve. (quote) "The valve closing can be heard as a sudden hiss on the flightdeck on climb out or during final approach at 2psi." (unquote)
(My copy is version 65, Feb 2013)