There can be quite a bit of “sound” when the disks are active, because they often are doing something. On the otherhand “noise” is another matter, so are you sure you are confusling one with the other? Anyway, if a disk WAS noisy, I would not expect the other to be so. the sound of both should be about the same.
I am talking about regular access and seek sound/noise. No weird other sounds.
However, I think the hard disks are way too loud! (WD REDs expected to be quiet).
I have a lot of old external and internal hard drives - not one is nearly as loud as my new REDs.
Maybe I’m spoiled from my SSDs, I don’t know. But seriously, the DL2100 should work pretty silent, regardless of file transfers, streaming, etc. Could also be a problem of the whole NAS construction, I don’t know.
Is there any hardware/software setting for “automatic acoustic management” for the HDs or the NAS system?