Nicely done, James!
I'm familiar with that sound you heard....Last weekend I was on a night hunt with a friend. We noticed a big pig in a trap about a 150 yard away and decided to swing by after checking all the common kill areas on the property. We walk up to the trap and a 200+lb sow charge us - it made that growl you're referring to. Every time it charged, it growled.... It sure is hard to keep your feet planted when that happens!
It takes 43 muscles to frown and 17 to smile, but only 3 for proper trigger squeeze.