The Media Generator has two cloud presets under NoiseGenerator.
Drag one to he timeline and strect it to the duration you need.
Double-click the media and hit q to turn on looping playback. Start playback and hit the edit nedia button (topmost of the 3 buttons on the right edge of the media).
At the bottom of the dialog box that opens is the keyframe controller.
Click at the right end of the keyframe timeline and make an adjustment to one of the variables such as Offset (making changes will automatically create a keyframe). The looping playback will show you how the media changes over time.
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com