I'm thinking perhaps my drive is particularly noisy. Or perhaps 3TB drives spinng at 7200 rpm are all going to be this noisy.
I'm going to try to confirm with Apple if the HDD is supposed to spin all the time, but at the moment it seems to be the case. Assuming that is the case then If you have a backup disk or something just hold it at arms length in front of your nose and see if you mind the noise. I might look up the specifications of my drive and see if it specifys the decibels, they often do, then compare to smaller slower drives...etc
Assuming you can return yours then perhaps just wait and find out, mine is just one data point / opinion...
Is your processor an i5 or i7?