What would cause a 738 to creak like a submarine at depth?

[Jetdoc]

I would take a wild guess and say that the noises are coming from the riser ducts that take the conditioned air from the manifold to the main distribution duct in the ceiling. Most Boeings have atleast 2 ducts per side. I never worked on a later series 737 but I believe that they would still be located just forward of the wings.