i tried this for a bit of fun and was really surprised by the difference it made so dont laugh :-) but cheapo rubber car mats work wonders when you line the insides of your case with them. i purchased five at 0.99p each and this covered the two side panels and all the spare drive bays top and bottom etc inside my case.
i also cut away the metal covering my case inlet and exhaust fan mounts. i have seven fans in total inside my case (2x 8mm case front and back + a cpu, graphics card and power supply fans) and can hardly hear them now at all, well worth a fiver and an afternoons work in my opinion.
the downside is the rubber adds a lot of weight to the case this help cut down vibration but its maybe not a good idea if you move it around a lot. i thought my case and cpu temps would go sky high but they remain in high twenties for the case and 35- 50 for cpu. here is some links if you dont mind rolling your sleeves up and getting stuck in click here click here