Maybe the noise come from an damaged drain valve that acts like a "flute".
AMM:
DRAINS - DESCRIPTION AND OPERATION
1. General
Drains are installed in the fuselage structure of the aircraft at or near to the lowest points in an area. The drains let water and condensation drain out so that they do not cause corrosion.
2. Description:
Drain valves are installed in the fuselage shell. When the cabin pressurizes, the valves, preset to operate when cabin pressurization reaches 2 psi (0.1378 bar) , close to prevent loss of cabin pressure. As the cabin pressure decreases below the preset value, the valves open (under spring tension) to allow any water that has collected to drain away. You can remove the drain valves, for inspection or replacement, with an allen key from outside the aircraft.
In other structural components (doors, door frames, stringers, brackets and fittings) holes are drilled at the lowest points to make sure that water and condensation drain to the drain valves.
FWD Area:
