It's difficult to translate this sentence exactly because you can't just map the structure of the English sentence into French. It's one of many cases where English and French idiomatically take a different part of the meaning to serve as the verb.
The most common such case is expressing movement, where English tends to express the manner through a verb and the direction through a particle, whereas French tends to express the direction through a verb and the manner through a complement. For example:
(en) He climbed up the stairs.
(fr) Il a monté l'escalier en courant.
To come back to the specific question, “The snake is hissing at the man” conveys that the snake is antagonizing the man, and that it is doing so through a hissing noise as well as an implied stance. The preposition “at” does the main job of conveying the antagonism. In French, we would tend to use the verb to express the antagonism, and optionally a complement to indicate the noise. French requires a slightly stronger commitment to the nature of the antagonism: there is no exact equivalent to the vagueness of “[make a noise] at”. Thus here are a few possibilities:
Le serpent se tourne contre l'homme et siffle.
Le serpent confronte l'homme en sifflant.
Le serpent menace l'homme en sifflant. (propoed by Rémi)
If it's not critical to mention the hissing, it can be omitted in French.