Humming sound after sound effects

I am currently using audacity to make voices for an animation movie. I use the sound effect/pitch change to make the many different voices. I am using one project for each act with imported sound effects. While making the voice changes I have noticed that some of the voice changes have a humming sound. I am not sure what is causing this. I have a male an female voice which sounds okay but some of the other voice changes get this humming sound. One in perticular is a voice of a dwarf with a high pitch.

Try using the “Sliding Time Scale / Pitch Shift” effect instead.