The rattlesnake is native to the entire Southern California area, but lives primarily in foothills and desert environments. I have heard its rattle several times up close and it is indeed very loud. The snake almost never bites unless provoked, so if you see one maintain a safe distance. Although the bites are rarely fatal, if you or one of your party are bitten seek immediate medical attention.
The California Department of Fish and Game reports that most bites occur between April and October, when the snakes are most active. There are about 800 bites per year reported throughout California, and 1 to 2 fatalities. Follow the advice on the California Department of Fish and Game page if you or one of your hiking companions is bitten.