Basic:
* Loud
* Noisy
* Blaring
* Roaring
* Thundering
More descriptive:
* Deafening
* Earsplitting
* Piercing
* Shrill
* Cacophonous
* Booming
* Rumbling
* Clanging
* Screaming
* Howling
* Bellowing
* Crashing
* Explosive
Figurative language:
* As loud as a jet engine
* Like a bomb going off
* Could hear it a mile away
* Vibrating the floor
* Shattering the silence
Specific to the source:
* The engine roared
* The music blared
* The crowd cheered
* The siren wailed
To emphasize unpleasantness:
* Irritatingly loud
* Distractingly loud
* Painfully loud
* Overpowering
To emphasize pleasantness:
* Energetic
* Exciting
* Lively
* Vibrant
The best way to describe something loud depends on the context and the effect you want to create.