A snarl is a facial expression, where the upper lip is raised, and the nostrils widen. It usually indicates hate or pain.

Snarling uses the levator labii superioris alaeque nasi muscle.

Snarl is sometimes used to mean an angry-sounding noise, e.g.:-

"In the dark there was a snarling noise" (Lord of the Rings by Tolkien, in the Barrow-wight chapter).

"the snarl of chainsaws".

This usage likely started by people misunderstanding the expression "growling and snarling".