The likely causes are fans & HD.
Clear all your fans from dust and do a Scan Disk to check if your HD has any bad sectors.
Or go to the HD manufacturer's site and run the diagnostic test.
I cant quite tell from where specifically, but my tower has been making subtle rattling noises. It's hard to describe, it's somewhere in between a rattling and a buzz. I think I keep my computer pretty well maintained, but this confuses me, heh. I have a fan mounted in the lower front of my tower which is turned on manually, when I do turn it on the noise stops. I would leave the fan on constantly but I leave my computer on overnight often and it is a hard choice picking out which noise is least bothersome, the fan or the rattling, haha. I was just wondering if anyone knew what this was and how I could fix it? Thanks