Here's why:
* Verb: A verb describes an action, state of being, or occurrence. In this case, the bear is *making a grumble*.
* Noun: A noun is a person, place, thing, or idea. "Grumble" on its own could be a noun, but in this sentence it's being used to describe the action of the bear.
Let me know if you have other sentences you'd like to analyze!