Maybe pushing toothpicks into the flyback gaps between core and donut will help.
I have also noticed the halves of flyback cores sometimes have tiny squares of rubber where they butt together.
The RCA sets of the early 80s had this issue occasionally and we would disassemble the flyback core to add or replace these rubber pieces, often with fiberglass tape. Sometimes just tightening the nuts on the core retainer "spring-hoop" would eliminate the squeal.
BTW, I have not heard this sound from just a TV anymore, I have near-constant ringing in my ears and hear it all the time
