Some other options..
There is a way to put a mic and speaker in the chassis,
and have the recording analyzed - and generate the 'negative image' of the sound..
Wave breaking up wave, should reputedly result in silence..
Or..
Do some carpentry, make a box for the computer with sound proofing materials as padding.
There are some expensive materials, and some cheaper (maybe just rockwool?).
Still you need to fan out hot air from this box, but you can do so with i.e A low noise(not to costy Papst fan.
Even better is to ventilate to the outside!
One exhaust pipe, with an exhaust-fan on the outside of the building/cool basement etc..
And one intake hole that should be same size or bigger.
Might even add a smal pipe to give CPU cool outside air too.
U can also get soundproofed pipes for venting.
-E