How do you define loud? In deciBells.
My definition: When your eardrums meet in the middle, you KNOW it's loud.
Now to seriously answer your question.
One of the most convenient 'cannon' you could own is the Winchester (or replica thereof) 10 ga. blank cannon.
Uses blackpowder blanks.
Genuinely loud, however, would be a surplus cannon/bomb simulator that uses propane. A friend of mine used one QUITE successfully in a parade where the amount of noise asked for was very loud. His wife convinced him to NEVER do it again, simply by handing him a mirror. When he saw the blood dripping from ears, eyes, nose etc. it was a moot point.