Interestingly, "snarl" and "rattle" are both etymological equivalents. "Snarl" comes from the German word for "rattle." Both words are onomatopoeic, meaning that the words imitate the sound they stand for. However, despite their similar meanings, the connotations are slightly different. "Snarl" implies an animal baring its teeth and growling. This is an appropriate word for the saw for a couple of reasons. First, Frost personifies the saw later in the poem; it "knew what supper meant." Second, the saw had teeth. It was not a chain saw as we might envision modern log-cutting projects using. The poem calls it a buzz saw, which is a saw with a large round wheel full of teeth that turns by a pulley. See the link below for pictures and an excellent discussion of what the saw Frost wrote about in 1916 may have been. Therefore, the "snarl" of the saw would be the sound the saw made when in contact with the wood, when the teeth were in use.
The word "rattle" is the sound of something shaking or vibrating. When the saw was running but not engaged with wood, it would make a shaking, vibrating sound. "Rattle" also has an ominous connotation associated with a rattlesnake, which makes a fearsome, death-predicting sound. In that way, the word "rattle" foreshadows the tragedy that occurs later in the poem. Thus the sounds filling the yard would be alternating snarls and rattles; snarls when the wood was being cut and rattles when the saw was awaiting its next log.