We often see zombies make some kind of snarling or growling noise as they attack their victims. However, I do not see this happening if zombies were real. In order to make noise, you have to be breathing, which is not possible since zombies are dead. Therefore, it would make sense for zombies to not make noise of any kind. The one exception would be the snapping and cracking their joints would make due to the effects of rigor mortis.