Air Leak Noise
The purpose of the keel beam drain is to eliminate fluids from the bottom of the fuselage. Hopefully the fluids are merely water from condensation on the interior of the aircraft skin.
On the A320 series you will find a grate on the floor of the flight deck, just after you enter from the forward galley. Under this grate is a basin that collects fluids that may come from the galley area. The fluids could be due to leaks in the galley or more commonly, rain coming in the forward entry or service door when the aircraft is on the ground.
Since this basin leads to a keel beam drain, one may hear the air leak noise emitting from below this grate.
The engineers I have worked with over the years have patiently answered many of my questions regarding the Airbus. If you have the inclination to learn more about the Airbus, see if you can get them to share a copy of the Aircraft Maintenance Manual. It will provide you with more of the system details than even the most technically adept pilot would need. Try to get the AMM specifically for your company’s aircraft. There may be substantial differences in systems of aircraft within the same fleet, due to hardware and software upgrades.
Sincere apologies for the thread creep.
Best regards,
The Kid