I have the 1x8 TB version in my desktop, which is muffled, and I have 4x 6TB in my Synology 1019+, which is in a closet. It is known that Ironwolf are "slightly" louder than WD Red, but we are talking about 1-2dB maximum.

Of course, the question is what you mean by "unbearably loud". I myself can say that under a long full load, a slight buzzing can be heard, but no roaring/rattling (if I leave out the cabinet or the housing cover).

So I would rather say that one of your hard disks has a defect. That's why I would recommend a SMART test via Synology DSM. Hopefully it will show the culprit.

I had a DoA once before, because the hard drive was not doing well in the post. So it would not be surprising.