...the sound is essentially a buzzing sound at the bore end of the barrels as opposed to a rattle.
Just a guess,,Vibrating/buzzing like that ,,I'd look for a completely loose rib at the muzzle going back a couple of inches perhaps. Top,,bottom,,could be either or both. Should be able to see it at the muzzle if the solder joint is broken away between the rib(s) and the bbls. See if you can push (gently) the rib(s) up & away from the bbls at the muzzle.
Beads are sometimes removed by levering them out with a sidecutter type tool. But it can put alot of upward force on the rib if it's done carelessly & you have a stubborn bead sight in place.
They can also get carelessly snipped off at the shank too that way leaving the stubb/shank to be removed. The tapered stubb can end up falling into the void in it's attempt to be retrieved and become a rattle problem. Rolling back and forth as the barrel is tilted end to end.
Hope you have success in working out the problem.
