I had a similar noise problem not so long ago and thought it was the HDD but the Evesham engineer found that the real cause was a failing mobo fan (on the north-bridge). If you are sure its the HDD don't hang about, take the advice already given above.
my 120g maxtor drive was doing that from new,i had the bios set to return the pc to its last state when pluged in, when i changed it to power off it stopped.im not a expert and it might be coinsidence but worth a try.
I have to agree that the most likely cause is not the HDD. So make absolutely sure, by unplugging the power from it, and then testing again.
If it is the drive, then it will almost certainly need replacing. Try setting SMART to enabled in the BIOS, then getting a free trial fo Active SMART click here - this will tell you if any HDD operating parameters are out of kilter.