Screaming/Growling isn't actual singing. Its just screaming or growling. The guy from Avenged Sevenfold wrecked his voice from not doing voice warm up's. You dont need training to learn to warm up your voice. One piece of advice I would give anyone who wants to "learn" to sing. Join a choir. I did when I was in high school. I'm in an alternative rock band trust me no same in trying to become a better singer.