Whose howl's his watch
This is part of an extended metaphor in which Macbeth describes a wolf stalking around, looking for victims and howling “withered Murder” when he locates one. “Watch” in the sentence is referring to a “watchword,” a rallying cry used by generals before a battle like “freedom” or even “for king and country.” In this metaphor Macbeth is saying that the howl before the wolf kills its pray is its watchword.