From TalkBass Wiki
1) Primarily a pronounced midranged (can vary low to upper) grrrrrrrrr-like tone. Often enhanced by 600Hz boost, aggressive technique, playing near the bridge, and accenting the bridge pup.
2) Gallien-Krueger defines growl as-a small amount of even order harmonic distortion (UrbanAssault415)
3) strings can produce a "string growl" exhibited during slides resulting from the strings lightly grazing the frets - becoming string rattle when the action is too low.
