You could do what I did - Invest in a inexpensive noiseless computer case coated with sound dampening foam, or buy the foam independently and apply the material to the biggest surfaces inside your case for less but still effective sound dampering.
I got
this one for about 120 bucks. Noise reduction in this case was exceptional for the price in my opinion - but then again, I went from a water cooled rig with a dual radiator powered by two high performance 120mm's to this one with stock case fans (which are quite good) and a high performance air cooled sink from Zalman.