Is there a snake in my dog food bag?

I had put my dog food bag outside a few days ago and when I brought it back in, about 20 minutes ago we had heard a rapid patting sound against the inside of the bag. My mom said that might be the tail of a rattlesnake. We are now scared of the bag, and we don't feel comfortable trying to zip it up, or bring someone into our house to take a look. Can anyone give us some advice?