If you can do a horse lip-flap (similar to getting a buzz on a tuba, LOL), well, it's just about the same thing with your tongue. The middle of the tongue stays in place while the tip just flaps against the roof of the mouth, just behind the upper teeth. It's just a sound effect. Most people learn to do it as a baby.
Now, just for giggles and FYI, I can even do something I call a "double-flutter" where I get the back of the tongue (about halfway between the "growl" area and the tip of the tongue, going alternately with the front, with the air stream narrowed almost to a hiss. It sounds very stuttered in the horn, and I've never found a use for it at all, but there it is. Somewhere along the way, I actually figured it out early in my playing. I can actually growl, double-flutter and horse-flap the lips all at the same time. (It sounds gross and it's absolutely useless, LOL!) Some of the aptitude for this sort of thing comes does come from native language, but some of it may also come from an infantile predisposition to making sound effects. I've noted studies where it was found that male babies are more prone to this, and also more prone to making sound effects in animated conversation later in life.
There's a second type of growl sound where you actually hum a note that causes a harmonic "growl" or distortion-effect with the lip-buzz in the horn. It's a really dirty sound, great for bluesy stuff. I have a hard time with it. I had a friend who was just a natural master of it, so, again, aptitude for the freak-playing sound-effect stuff is a big factor in how effortlessly a player can pull it off. Listen to a lot of George Rock and even older 20's and 30's radio-show stuff to pick up most of the novelty sound-effects that might show up on a chart sometime, LOL.