I just took one of these apart and they are quite easy to take apart. I would open it and inspect it further. This machine has a tiny fan with huge vents that facilitate dust and debris from getting into the fan and its chamber. Get in there with a can of compressed air clean things up. If it's your HD (the only other moving component) then I suggest you back-up and get a new HD as soon as possible. But, I'm leaning to the fan as the culprit for the noise.