Swoop

I have a 20 ga montefeltro that developed a rattle over the weekend. It is not the bolt rattling, but is instead the long tube under the forearm that I believe houses the inertia mechanism. I can remove the forearm, hold the bolt still and give the tube a tap and it has a very noticeable rattle. Same goes for when the forearm is on and tapped, and I’ve never noticed it in the 15 years I’ve owned the gun. Any thoughts are most appreciated.