If you notice a crackling noise, it's possible that debris, like items made of paper and plastic, have been sucked into the unit and wrapped around the fan component, causing this odd sound every time the fan turns on. The debris needs to be cleaned out to prevent the fan motor from burning up.
Another likely culprit is your air ducts. Older duct work could fall apart, which might cause a crackling noise as the duct work vibrates with the running of the unit. Other duct work might cause a similar noise if it isn't secured adequatley to the structure of the house, leaving it free to rattle against various items within the attic or crawl space.
Trust the Experts to investigate and make the proper diagnosis for your HVAC system.