I just changed the filters in my A/C closet a couple of days ago. While I was down there (it is an upflow system) I made it a point to clean up as much dust and line that I can.

This ;morning I hear a crackling sound so I investigate and find that the coils are thickly frozen up. It's my understanding that it means a dirty filter or low freon. \

What are the other possibilities?

Peace