I just purchased two of the 200GB 7200.7 models within the last month and am experiencing the exact same problem with both drives. One of them does it louder than the other one, but they both do it CONSTANTLY when they're not being accessed. Mine start after about 10-15 seconds of inactivity. It drives me up the wall because though it's relatively quiet, is distinctly audible over my computer's fans. Considering the entire reason I purchased these drives is because they're supposed to be quiet, I'm a bit peeved.
I emailed Seagate tech support about the problem this evening, and am waiting to hear back from them to see what they say. Every drive diagnostic utility I can throw at these drives (somewhat limited, because I'm running MacOS X) seems to think there's nothing wrong with them.
I'd appreciate any information that any of you may have on this issue, and will post the reply I get from Seagate Tech Support as well.