I've got such a "fellow traveler" inside my fork -- I can't even tell which blade for sure. I always assumed that it was some slag from construction, but it's possible that it's a small piece of gravel. If it's ferrous, then you should be able to coax it around, or at least get it to stop moving, with a good strong magnet. If there's both an inlet and an outlet for air (my fork blades each have a small hole near the top, and near the bottom) then you might try the vacuum cleaner with a crevice tool. If you can get it at least opposite a hole, you might be able to crazy-glue it into place so it doesn't rattle.