"To thump", as defined by [Merriam-Webster](http://www.merriam-webster.com/dictionary/thump) is:

>to hit or beat something or someone and make a loud, deep sound
>
>to beat forcefully
>
>to walk or run with loud, heavy steps

Think of a male gorilla, thumping his chest.  Basically, it means to make a whole lotta noise, which is rather rude to the snakes if you're thumping or stomping around in their territory.