I have the same "problem" - I have replaced 2x 1 TB disks with 2x 12 TB Ironwolf in the DS218+. The things are unbearably loud, when accessed I measure 47db on the NAS. Without access, they still scratch regularly and audibly, just less quickly. Since I can't put the NAS away, it's extremely annoying.

The WD Red have 5400 rpm. The Seagate IronWolf (>4TB) 7200 rpm.
This makes them faster but (theoretically) also louder.

But I personally find my 10TB IronWolf quiet, even when reading/writing.
The old WD Green 4TBs were really loud in comparison.

Either way, if the hard drive is displayed in good health in the NAS, it probably is.
Backup is still a must for important data ;-)

Have you had the hard disk checked with a test tool to rule out possible errors?