You are correct. I should have said, 'different kind of rattle sound.' 'rattlesnakes and their rattle sound cannot be mistaken.' Chicken snakes don't have rattles. They just shake their tail making a rattling sound.
Cottonmouths also shake and rattle their tail when threatened.
Don't forget Copperheads and rat snakes also shake their tails. It is best to know your snakes.