Hi, I recently bought a Japspeed back box and middle section from a guy for £100 because the rattling tail pipe was driving him mad! It's like new. The issue seems to be the insert in the tail pipe that is tacked by weld on the bit furthest in and relies on the rolled flange on the outer end to hold it in place. It was this rolled flange on mine that was causing the inner part to rattle, Three discreet tacks of stainless weld between the outer and inner parts and a polish and it's fine. Did both just to be sure.
N.B. Any welding needs to be stainless, ideally TIG otherwise you will get rusting on the weld.
Hope this helps and you get it sorted.