scratchy noise when stopping playback

I was listening to and editing a single mono vocal track. I would stop and cut out breath and lip sounds. Somehow I touched something and created an envelope, got an error message, and then finally stopped the playback. I fixed the envelope so the blue lines are gone. However, I am left with a tremendously annoying scritchy noise when I stop playback (to cut or apply an effect, for instance). Whether I press the space bar or use the mouse, every time I stop playback I get a terrible sound in my ears!

How did I do this, how do I turn it off, and how do I not ever do this again? I’ve been working with Audacity for at least 6 months and this hasn’t happened before. Sorry to ask, but I couldn’t find this question or an answer in the forum.

Thank you much!