Imac 27 I5 WD Black Caviar HD Grumbling noise?

I have had this computer for almost 2 weeks.Tthe HD does make a low grumbling noise when seeking and I guess writing. It's not a screeching noise, but a low grumbling. I read a few reviews of this HD itself where they did mention that it did make more noise than other HDs, but had great performance. Is this normal or would replacing it result in any improvement. Are Imac still shipping with this grumbling type of hard drives or would a switch or return result in a quieter machine. The noise isn't constant.
Thanks