Some cats howl when they are hurt. Gemini only howls when she is frustrated. Having proven herself indestructable, I know that when I hear the howl of this cat, there is something she wants that is not accessible to her. For instance, the elder cat in my household occassionally goes outside and right now she is howling at the door to join her.
At first she stood there and howled. Then she came and found me. Shortly after she noticed the other cat outside and went romping over and pawded at the back door for awhile and then began to howl again.
She periodically gets up on railed area on the second floor and howls. I am not quite sure what she wants to do there or if she gets confused but it seems to be a regular thing that she does.
Then there are the times when she doesn't howl but is just persistent. She really wants to play with some blueberries I am attempting to eat, but I keep moving them. I have never seen a cat who so thought human food was just another play toy.