Nope, definitely not the HD.
I have a feeling something was doing a check, unbeknownst to me, and it was working the CPU. As soon as I logged off here and went to another site, it quit cycling.
I did open it up and it's pretty clean inside so I let it be.
At first I thought I'd never been inside this box. Then I remembered I put the new RAM in it when I got it.
All seems to be OK now.