A tumult is an uproar or noise, an angry demonstration or disturbance or a conflict of emotions in the mind.
Tumultuous is disorderly, noisily agitated or making a tumult. If you're being those things, you'll be acting tumultuously and demonstrating tumultuousness.
Waterfalls are a tumultuous torrent of water.
Can you imagine the tumult of emotions the pixies experienced when Gary knocked on their door? He only tapped gently, but to them it probably sounded like a tumultuous racket.