Intensity:
* Boisterous
* Clamorous
* Decibel
* Earsplitting
* Harrowing
* Rumbling
* Roaring
* Thundering
* Vociferous
Volume:
* Blaring
* Cacophonous
* High-pitched
* Resonant
* Shrill
* Strident
* Trumpet
Other:
* Brawling (for a chaotic sound)
* Obstreperous (for a disruptive sound)
* Raucous (for a rough, harsh sound)
* Tumultuous (for a noisy, confused sound)
The best synonym for "loud" depends on the specific situation and the type of loudness you are trying to describe.