To my mind, a dull thud tells you that you not only have separation, but a serious problem in that a good bit of rusting has already taken place. I'm with Jimmy and Oscar concerning a pinhole or two. If they ring well, there probably isn't a problem with oxidation, yet. It would seem to me that if heavy oxidation hasn't already set in, an old cure for "ghosts" would stop this potential. Put a small hole in the lower rib,[under the forend], remove the front sight, then heat the barrels to 225 or so for a little while to remove any moisture. Once dry, inject a little boiled linseed into both openings and move this around by tipping the barrels back and forth. When the void is well coated, lean the barrels muzzle up, lower rib down, and any excess will drip out. With the linseed dry, oxidation is either stopped completely or very nearly so.
In the case of early separation,[as opposed to a couple of pinholes], it can often be well seen with the use of a very thin feeler gauge run along all four solder joints. If the feeler slips under the rib, true separation has set in, and only a relay will solve it.
Jim
