Are you sure it comes from your HDD? Do you happen o have coolers installed on them or in the proximity? It could be them. If you are sure that it's one of your HDDs, I would simply unplug one....boot. If the noise stops...you know which one it is. If you in fact discover it is one of your HDDs BACKUP FOR YOUR LIFE! It could go wrong any minute.
I would back up NOW because you dont want to run all these tests to find out it is your HDD and then it's crashed completely and you had no time to back it up.
Of course this is mearly a suggestion
I think you should backup your data. I never used those new SATA Hard disks, I still like the old fashioned IDE ones. Untill they prove their reliability, I won't use them
You might want to disconnect all the hard disks and all the fans and run the computer for a small period of time and see if the fan of the Power supply is producing such noise.