Over time, the transformer core adhesive starts to break apart, maybe some large heavy duty hose clamps could help.
https://www.qsl.net/kf8od/ccore11.jpg
https://www.qsl.net/kf8od/ccore7.jpg
https://www.qsl.net/kf8od/ccore11.jpg
https://www.qsl.net/kf8od/ccore7.jpg