hmm...I'm not an electrical engineer, but I've found that buzzing is often from a transformer. There are a couple transformers inside the machine, one of them is powered continuously (it feeds the "power on" button) but the others are only powered after you push the "on" button.
What you might have to do is have somebody else push the button while you try to figure out where the noise is coming from. (if you haven't done that already)