You can't, short of watercooling.
I hate the noise this card makes but fortunately I don't run a 6990 rig at home (two of them at datacenter). Absolutely could not bear it 24/7 in close quarters, I would say it easily makes about 80dB at 100% speed.
Of course, one option is downclocking the cores which will slightly affect the mhash/s. Downclocking memory as well.
Already downcloacked memory.
I was wondering because I saw some ppl saying that they run their cards with fans at 60-70% and lower temperatures.