I have a new extreme rattle on all open strings and fingered notes. I have looked for anything loose or open seams and can't find the culprit. There was something rattling around inside the bass and I shook it out- it was a small cork disk. Could it have been seated around the end pin on the inside? It was brittle and broken. If this now missing piece is the problem- how do I get a new piece of cork back in there?