The "grrrrr" you heard was most likely the crow chocking in its own blood (blood in the trachea), ie: trying to breathe. It's actually like gargle sound when air is mixed in with blood in their nose and trachea (air passages). A few critters I have shot (squirrels, Groundhogs, and a possum) make that sound when that condition develops after getting shot in the head. It sounds like a "low growling" but it's them chocking before they expire.
I did have a Groundhog really growl at me though when I got close to it after it was paralyzed or something from the shot and wasn't going anywhere but still alive. I backed up and he stopped growling. I advanced again and he growled again. It was the same as a cat when it's pis*ed - that deep growling coming from the back of the throat or chest (not hissing).
