|MadSci Network: Astronomy|
I think what you are seeing is the sound suppression system. It was found early on in the Shuttle missions that the tremendous noise from the solid rocket boosters could damage the Shuttle. To absorb the noise, valves are opened 6.6 seconds before liftoff which release water from a nearby tower. The tower contains 300,000 gallons of water which gets dumped into a cavity below the launch pad. The water absorbs the sound from the boosters and dampens the vibrations, making the Shuttle liftoff safer. It lacks subtlety, but it works!
You can read more about the sound suppression system at Space Shuttle Reference Manual, which has a huge amount of info about the Shuttle.
Try the links in the MadSci Library for more information on Astronomy.