Many hard drives these days, but especially Seagates, tend to make very loud (or at least annoying sounding) idle test runs. It depends on the hard drive model, but it can be described as anything from "chattering" to scratching all the way to small clicks or clunks that occcur every minute, or even more frequently. Believe me, i've tried several Seagates, and they all exhibit this "issue", although it IS normal operation. They drives do it to prevent the head from sticking (supposedly) and to keep reliability up.
The only drives that I can recommend that don't make these strange idle sounds are Samsung hard drives.