hard drives, especially the newer ones, aren't that loud, even when going full out.
I'd guess that either it's a VERY loud (probably caked with dust) fan, or your CD/DVD drive is spinning up. You can easily replace a fan if you want with a quieter one, or just open the case and blow all the dust out of it, making your fan more efficient and not running as often or as hard. your CPU should have small cooling fins on it, topped with a fan. Get a can of compreseed air and blow all the dust out of the box paying close attention to the CPU and the cooling fins/ fan. Also check the air inlets on the back of the Box, make sure they aren't jammed up with dust.