My money is on the bearings needing to be replaced.
If the spool end shafts were bent or out of alignment, you would feel that when reeling in.
When you cast, and you've removed the level wind drive mechanism, the only bits moving are the spool and the bearings; and when they're shot, bearings make a hell of a noise.
Put new bearings in and I bet the noise goes away.
There is another possibility; although it's very rare. Make sure that the ratchet (clicker) is releasing properly. If it is making even slight contact with the cog on the spool; that will make a noise when casting too.