Hi John,
Everyone else has the right idea, I would try tapping the hoops down first and see if that does the trick, if that doesn't work try pounding a thin cedar wedge in between the staves. If it's a visible worm hole or if you decide to disassemble the barrel, an oak spile (tapped into the drilled out worm hole) will work wonders. A spile is basically a glorified fat tooth pick. One other idea if you disassemble the barrel is to put flagging (AKA: cat-tails, bull rushes) in between the staves, they swell up tremendously and should work great.
~Heidi
Black Swan Cooperage, LLC