How to get rid of "buzzes"

Hi Folks,

I processed the speaking voice of a woman into Derek x 3 with a transpose of -10 with the volume set at halfway both in the plugin and my DAW. However, every so often I get "buzzes" when he's speaking. Is there a way to remove these? I have Audacity but not sure which the effects to apply?

Many thanks for any help you guys can give

Andrew