Unless you can describe the noise better or, better yet, record it and post the recording, we can only do one of three things
1) Agree with you that it is inappropriate
2) Agree with the seller that it is normal
3) Suggest that you go listen to some other Hitachi drives, if you happen to know anyone who has one.
Is K55 the model of the hard drive or the model of the machine that has the hard drive in it?