Depending on the direction of wobble, it is most likely loose on the back of the barrel loop. This was traditionally tightened by peening up the forend iron on the internal surface but is now more usually done by building up the bearing surface on the forend iron with TIG or laser weld and then filing to fit.
It is a very simple job for a trained experienced double gunsmith, should take less than an hour.
Beware, welding the forend iron will lose you any CH colour in that area.
Also check that your forend barrel loop hasn't come loose in the barrels, it happens!
