Static and Feedback Added to Parts of Audio After Mixing Down Tracks

I recently started using Audition to edit audio for my podcast. We still record in Audacity since I'm the only one to have access to Adobe. I've found that after mixdown down the 5 tracks, feedback and static are added at one or two points that weren't there originally. I went in to remove them and then mixed it down to export (as an mp3) and it got added again. Any idea why it's doing that?