Zebras make a variety of noises to communicate including sounds that sound like barking, braying noises and snorting. Zebras are very social and communicate with not only noises, but facial expressions and ear movements.
Zebras that lay their ears down flat are signaling that something is wrong or that they expect another zebra to back off or fall in line. Baring teeth or widening their eyes are other ways zebras communicate without making any noise.
Grooming is another way zebras communicate and bond with each other. Zebras will gently bite each other to remove pests or loose hairs, which is a way that one zebra shows another it wants to be friends.