I'm guessing that the reviewer got confused between bels and dBA ratings. A sound level of 2.5 bels makes a lot more sense than 2.5 dBA.
Seasonic is quoting 2.0 bels at idle for its latest 120GB HDD.
"The Seagate USB2 Pocket Hard Drive is the world's first external hard drive small enough for consumers to fit in their pockets or throw in a backpack. Safely encased in its sleek round shell, its robust design and high shock tolerance enables mobility and makes it as convenient as a USB thumb drive, while offering an incredible 2.5GB or 5GB of storage for transporting or temporarily backing up a huge amount of data."
All that and more in Seasonic's latest announcement.