I’m honestly not sure, I’m just pulling stuff from various online sources to get an estimate. This YouTube video measures noise from a single 7200 RPM WD Red drive, and I have 2x 5400 WD Red drives, so I’m guessing mine will be similar, if not a little quieter. I’d measure it myself, but it’s next to 2 other PCs and fan noise drowns it out.
Basically, I want it to work under my bed. It’s a large, thick bed (king size purple mattress; so basically foam), so I can stick it pretty far in to get some flexibility on what “quiet” means. We also aren’t particularly light sleepers, so our threshold is probably a little higher than others.
That said, I’m using the stock fan for my Ryzen 1700 (Wraith Spire I think?), and that’s way too loud for a bedroom (40dba-ish?), and I can’t really hear my drives over the fan unless I get really close. The spec on the drives say something like 20-30dba for my drives (I’m guessing that’s right next to the drive), and I think that’s quiet enough for my room, especially if I can dampen vibration a bit. But since the drives will probably spin down, I’m looking for fan noise around that level or lower, if I need a fan at all.