It could also be from the interior mat that lays on top of the polystyrene material is making contact with it now and the rubbing causes it to make noise.
My rattles have been far less than I had expected. I feel as if the chassis bracing I added may have perhaps reduced the chances of rattles, but some may consider that a stretch of the imagination.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano