Mine is just nonstop. Even the controller is making a weird sound when it vibrates, its like a grinding sound.
Going back asap.
That controller thing I think is normal, seems to have happened since the update, as the vibrations seem quite strong now so its rattling something?
I've also had the low buzzing noise, had the xbox for about 6 months now, and I've monitored it carefully and found out that it is constant, with no fluctuation or decrease during different loading periods. So I'm assuming its probably a wire clipping a fan, as if it had anything to do with the HDD it would probably change depending on if I was on the dash, or loading a game etc. and I'd probably get performance issues.
My warranty runs out next June so as I get no performance issues, I've just decided that I'll return it around March-April time and get it replaced just as a precaution.