A couple things which often cause pops, crackles, and 'rustling':
- Master Vol is set too high and tracks are clipping- but when you look at the individual track levels, they may appear to be normal.
- You've edited a track or 2 and added reverb, EQ, or compression, but that has changed the level of the wav file and even though it's not clipping in the mix, it's now 'ruined' because the editing added in the noise to the wav file itself. If this is the case, revert to the unedited original tracks, and try lowering the volume of the track in the editor before modifying the track with effects that would cause clipping in the editing phase.
- You're using a tube mic preamp that has the 20db gain boost engaged, or the output level is almost or near maxed.
Just a couple ideas, hope it helps.