The Carrier Roar is an ability that only the Carrier can use to alert other zombies in their area where they are. This is commonly used to tell the zombies that the Carrier is in the same place or near the survivors.
When in use the Carrier will take on a yellowish outline around them. All zombies on the map can see this glow through walls and in the dark making it easy for them to find the Carrier, they also spawn at the point closest to that Carrier.
For activate this ability,you will need have your Feed O' Meter full,and press the reload key,"R" by default.