Sounds like the noise must be generated by either a fan or a motor. Do you have a separate video card with a fan that's making noise? I've had trouble with my last two vid cards and fan noise.
You said in your 1st post that you're using two hard drives. Have you tried booting to just the startup screen with both hard drive's power disconnected? No harm would be done, and you would at least eliminate your drives as the issue.
And no, there are no other fans in the CPU other than heat sink fan you've already tried, so it's not the CPU.
