Why is my cat chewing on hard things?

I've noticed that my cat has been chewing on hard objects such as my ipod, drawer knob, and the corner of a shelf in my closet. I'm starting to get a little concerned because that doesn't seem normal to me. She is 2 years old, has no health problems that we know of, we adopted her from a neighbor. She generally is a fairly normal cat, but this is concerning me