Another Creepy Crawly

I went for a walk at the metropark this evening. I heard some birds in a tree a little way off the path and stopped to look at them through the binoculars. I was surprised to find this guy perched in the tree with them. Two birds were hopping on the branches all around him, while this snake made what sounded like a noise somewhere between a hiss & a rattle. I know it's not a rattle snake, and I certainly wouldn't have been able to hear it hiss from where I was standing, but it did make a noise. I thought this was a garter snake, but the info I read about garter snakes said they don't make noise. So, I don't know for sure what it is other than a nonvenomous snake resting in a tree.