Hey Tyler: Seems that everyone else here want to send you on a wild goose chase. There may be many opinions on what a "growl" actually is. But one possible technique can involve vibrating the area near the top/back of your mouth while maintaining something close to your normal embrochure. You can start by trying to imitate a cats "purrrrr". Try doing it without vibrating your tongue and only vibrating the top/back of your mouth. If it seems unnatural, just take your time and re-approach it over time. Not unlike an asian person having trouble pronouncing the difference between "r's" and "l's". Or whistling for the first time.

You have got to be kidding me. Kelsey has it right. But what he didn't say is to vibrate your vocal chords while playing. Or just hum while you play. Or just Grrrrrrrrrr. This Paul dude is the one sending you on a wild goose chase!