Marathon mixing session last night for my next CD.  I was using the express dynamics plug in in Acid Pro version 6.0 then rendering the mix to .wav.  When I opened it up in Sound Forge 9.0 for mastering the intro had crackling noise.  I went back to Acid and found that the noise wasn't present there.  I puzzled over it during dinner and after a beer I realized that track had the express dynamics plug in. I had read somewhere that adding compression to synth tracks makes them sound smooth.  Well the choice between smooth and noisy was easy for me.  I removed the plug in from all the tracks that had it, rendered it to .wav and opened it up in Sound Forge. Voila, no more noise.  Lesson learned:  Avoid the express dynamics plug in.