The aggressor will use visual cues along with what sounds likes backwards vocalizations. Even chicks will direct such signals at strangers.
The visual cues will often involve stretching the neck out and trying to keep head higher than bird the display is directed at. The the wings with be held out slightly with tips down to extend below contour of the body. The tail will be held about 45 degrees above the horizontal with the base flared to form a triangle.
To my ears the vocalization must consistently associated with aggression sound like gun-gun-gun with emphasis on first part of each syllable. The sound is typically louder than the contact call.
To get a better handle on things, I suggest you separate couple flock mates by a few feet and listen to their contact calls as they try to get back together. Then if you have a stranger, make so it can approach the original birds so you can witness the differences yourself. With a little practice you will be able to pick up on signals that say get out of my way from a good 30 feet or more.