When you remove the fore end, you will find a steel plate, the fore end iron, that is secured to the wood with two slotted screws. underneath that is a spring-loaded clip/tensioner, the fore end fastener bolt. You can see the end of this in the rectangular cut out in the plate. Looking into the cut out, you should see about 1/4" of the forend fastener bolt exposed. That bolt locks on to the barrel by bearing against a small lump that you can see in a corresponding position on the barrel. If one of the parts is worn or weak - the bolt, its spring, or the lump - the fit will be loose.
it's also possible that the fore end iron is loose or that screw is missing. Check and see.
Pete