No noise in original signals, but crackling noise when mixed?

I've been doing audio manipulation on my computer for over ten years, and I thought I knew it all...but this one is mystifying me.

I'm just trying to use Audacity to mix a backing track with some vocals I recorded. There is no noise in either signal. But when I mix them together, the vocals start to crackle!

The signal isn't clipping...I export the track and it's actually got a good amount of space before I have to worry about clipping. My only theory is that somehow the frequencies in my voice are interfering with the frequencies in the music somehow. I did the usual processing on the vocals to make them sound nice -- DC offset fix, level compression (4:1), and echo (0.01s with 25% wet/dry mix).

Any idea what I should try?

And while I've got the attention of pro audio people...now that I'm recording more of my own stuff, instead of just fixing already-recorded stuff, I'm ready to move beyond the microphone port on the back of my computer. I want a nice breakout box that Audacity can recognize. Any recommendations?