From Wikipedia Kennedy Space Center Launch Complex 39:
Sound Suppression Water SystemA Sound Suppression Water System (SSWS) was added to protect the Space Shuttle and its payload from effects of the intense sound wave pressure generated by its engines. An elevated water tank on a 290-foot (88 m) tower near each pad stored 300,000 U.S. gallons (1,100,000 liters) of water, which was released onto the mobile launcher platform just before engine ignition. The water muffled the intense sound waves produced by the engines. Due to heating of the water, a large quantity of steam and water vapor was produced during launch.
-PJ
Thanks. I think I remember seeing something like that in a show about the Space Shuttle.