[Jonathan Starr]

I have an Omnirax desk with a deep rack area on both sides under the shelf. I lined one of these areas with acoustic foam, top bottom and sides and moved the CPU back into the tunnel about 8 inches. This cut the fan noise at my listening position considerably (almost inaudable) and did not affect cooling since the front and back are still open and there is about 5" clear on each side between the computer and the foam.

The only issue is that the CPU would theoretically be somewhat unstable sitting on the foam; however the top of the CPU touches the foam at the top and that keeps it stable.