Kunal, I had the same problem on my 15" mid2010
I tried both to fasten and loosen the screws on the botton, I even tried loosen and fasten the screws on the harddrive inside the computer a couple of times. Nothing helped.
I tried to install a small 80GB harddrive, which I knew was very very silent when working ( the 1 from my white Macbook ), and the noiselevel is clearly amplified by the alu-box itself. I think the fastening-system in the alu-box is the problem.........2 screws and 4 pieces of plastic/rubber isn't good enough, apperently.
I swapped my stock-drive with an Intel 160 GB SSD a week ago, to get rid of that bloody noise, and it works perfect.......Expensive but good.