Is the joint separated and the rib loose? Push on it laterally just ahead of the "scarf" (diagonal joint) with the short section of rib over the chambers. Does it move? May be nothing wrong except separation OR solder-starved section original to the gun which collects moisture. Might not be apparent that it's loose or separated due to the fact that the rib is mechanically trapped and held down by the scarf. One possibility anyway; I'm sure there are other explanations having to do with flux residue.
jack
