I had a small rattlesnake get into my garage and my friends Pomeranian got bit twice in the face and we heard nothing except the dog yelping. The snake was less than a foot long with only a button "no rattle".
The dog survived with anti venin $5k but, the skin on its muzzle fell off (grew back thought).
Yeah. It's a myth that they always warn with their rattel. They usually do, but, not always. Sometimes they just strike.