strange sound from macbook pro

I've had this 2010 i5 15" mbp for about a month now and swapped out the hard drive for a Hitachi 500GB Travelstar right after I bought it.

I have not experienced any issues until today when I started hearing this sound (see attachement) ever so often. It is difficult to pinpoint where it is coming from.

Any ideas on what this might be?

 

fwiw, i think the issue was the hard drive. I took the hitachi back to fry's and exchanged it for another one and so far today, no issues.