I thought it was a fan at first so I checked each one and then I laid my hand on the drive and found the problem.
That's an excellent method of identifying the problem:
*Open the case - and make sure you can quickly access the insides
*First, listen to narrow down the location of the buzzing
*You should be able to lightly touch each component and you will be able to tell which one is causing the issue
It should go without mention, but DON'T touch the fan blades - especially on the CPU! Still, you should be able to touch the fan casings.
"A" likely culprit to your problem is the HDD, but I wouldn't rule out the GPU...possibly a fan.
Hope this helps.