I’m thinking 25DB is a hard cap, ideally under 20DB.
I think HDDs are typically around 5-10DB,
Um no. More like 20-25db at idle, up to 30 during heavy seek activity, depending on model.
I run 3x 5400rpm drives in my NAS, and the drives are definitely the loudest parts in the whole build, and are definitely noticeable in the office room.