You 'catch' the dog growling, then reward.
I use a clicker for this, as timing is important here.
Then added a que word with handsignal simultaneously.
In order to catch your dog growling....
i trained at my front door, where they tended to growl, and for one of them, they growl whilst playing tug of war, so captured it there.
My hand gesture is the signal you make for talking, or glove puppet for want of a better descriptor. ITs discreet, door knockers cant see me doing it behind my back at dogs. VERY effective.
And my first dog the GSD, taught my rottie, and both of them are now training Brian for me! sweet.
And you know how one starting up, triggers the others, that is what happens. So the noise level creschendos. Sounds awesome!
The wagging tail is a dead giveaway to those who can 'see'.