garageband crackle

please any ideas why after 2 years of working with Gb is there still a random crackle that just appears wherever and whenever it feels like it, not neccessarily on the original song but turning up on the mix and as i say totally random places ruining evey mix. I have to do as many mixes as it takes just to get one take where im lucky enough to miss theis noise popping up. this is no way to run a studio and record an album. Surely there must be one person with GB that REALLY understands this common problem by now.??